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5.F Pure steam systems

Here you will find answers to the following questions:

  • What are the theoretical principles of steam and pure steam?
  • What quality requirements is pure steam expected to meet?
  • What is the procedure for generating pure steam?
  • Which factors must be taken into account when planning and designing loops for pure steam?
  • What must be taken into account for equipment used in the procedure?

 

5.F.1 Physical principles

Steam is a very complex and extensive topic that is associated with both GMP-specific and health and safety-related risks. If you require further information, you should refer to more detailed literature on the topic or consult a suitably qualified expert.

Steam is a medium which is primarily used for the transfer of heat. Steam can be easily produced from water. To generate steam, the water is simply heated until it evaporates. The reason why steam is used for transporting heat energy becomes clear if we compare the amount of heat energy stored in the same mass of water and in steam.

1 kg water at 100 °C:
1 kg steam at 100 °C:

417.5 kJ
2,675.4 kJ

This significant difference in the heat energy (enthalpy) contained in both media - despite having the same mass and temperature - is a result of the heat of evaporation contained in steam. The heat of evaporation is the heat energy that must be applied to cause vaporisation, without causing a rise in temperature. To vaporise 1 kg of water at a temperature of 100 °C requires a heat of evaporation of 2,257.9 kJ. The temperature at which the water evaporates also depends on the pressure. For example, at an atmospheric pressure of 1 bar, water evaporates at 99.6 °C. If the pressure is increased, the evaporation temperature and the amount of heat energy contained in the steam also increase. Figure 5.F-1 shows the pressures, temperatures and heat quantities for saturated steam.

Figure 5.F-1 Steam table (dry saturated steam)

pe [bar]

t [°C]

h'
[kJ/kg]

Dhv
[kJ/kg]

h''
[kJ/kg]

v''
[m3/kg]

r''
[kg/m3]

0.0

99.63

417.51

2257.9

2675.4

1.6940

0.5903

1.0

120.23

504.70

2201.6

2706.3

0.8850

1.1299

1.5

127.43

535.34

2181.0

2716.4

0.7180

1.3928

2.0

133.54

561.43

2163.2

2724.7

0.6060

1.6502

2.5

138.87

584.27

2147.4

2731.6

0.5240

1.9084

3.0

143.62

604.67

2133.0

2737.6

0.4620

2.1645

3.5

147.92

623.16

2119.7

2742.9

0.4140

2.4155

4.0

151.84

640.12

2107.4

2747.5

0.3750

2.6667

5.0

158.84

670.42

2085.0

2755.5

0.3160

3.1646

6.0

164.96

697.06

2064.9

2762.0

0.2727

3.6670

pe Positive pressure
h' Enthalpy of water
h" Enthalpy of steam
r" Density of steam

t Saturated steam temperature
Dhv Heat of evaporation
v" Volume of steam

Steam can only release the stored heat of evaporation on condensation. This means that 1 kg of water at 100 °C can release a maximum of 417.5 kJ, while 1 kg of steam can release 2,257.9 kJ into the environment, in order to return to an aqueous state at a temperature of 100 °C. Furthermore, condensation is a relatively rapid process, and hence the 2,257.9 kJ becomes available within a very short period of time. In contrast, the speed at which water can release its heat energy depends on the temperature difference compared to the environment into which it will release its heat energy. The lower the difference in temperature, the slower the transfer of heat energy. Regardless of the temperature difference, this process is always slower than condensation.

As long as steam remains in contact with water, it also shares the pressure-specific boiling point of water (see figure 5.F-1). Steam that has the same temperature as the boiling point of water is called saturated steam.

Saturated steam which carries water droplets along in its flow or in which a low temperature has caused drops of condensation to form is referred to as wet steam.

If heat energy is applied to the steam above its boiling point, the temperature of the steam continues to rise above its boiling point. This steam is then known as hot steam or "overheated" steam. The necessary prerequisite for generating hot steam is that the saturated steam is no longer in contact with the water when the heat energy is applied. Otherwise, the heat energy would cause the water to continue evaporating. Although in a closed system, pressure and temperature would also continue to increase, the generated steam would be saturated steam. Hot steam can also be produced by reducing the pressure of saturated steam. As described above, the temperature of saturated steam at a particular pressure is the same as the corresponding boiling point of water at this pressure (figure 5.F-1).

If the pressure in a steam line is reduced (e.g. by a valve or an aperture), stationary expansion occurs. Since no heat is applied or removed in this pressure reduction, the enthalpy of the system remains constant (adiabatic expansion). In practice, the resulting temperature is calculated using the "Mollier Enthalpy Entropy diagram".

For example:
Steam with a positive pressure of 4 bar has a temperature of 151.84 °C. If the pressure is now reduced to 1 bar, according to the enthalpy-entropy diagram, the resulting temperature is approx. 130 °C. According to the steam table, the steam should have a temperature of 120.23 °C. There is therefore an excess heat of approx. 10 °C.

If the temperature of the steam is below the boiling point, small drops of condensation begin to form and the steam becomes moist. The dryness of the steam is specified in percent (e.g. 98 % dry). Depending on the dryness and the pressure differential, moist steam can also become overheated steam again, for example by a reduction in pressure. Steam can also be dehydrated using "steam dryers".

Steam can change its properties within a steam system. This shows that dry saturated steam in a real-life steam system is a purely theoretical case. The steam in this type of system will always have a certain degree of overheating or moisture, even if only slight. These factors must be taken into account in the design of the system.

5.F.2 Quality requirements for pure steam

Unlike ultra pure water, no requirements for pure steam are specified in the pharmacopoeias (e.g. Ph.Eur., USP), and no limit values have been defined for the chemical and physical properties of pure steam.

Specifications regarding steam quality are defined in several standards, including the following:

  • DIN EN 285 Sterilisation - Steam sterilisers - Large sterilisers
    This European standard replaces the German standards DIN 58946 part 2, part 3 and part 7.
  • DIN 58950 Sterilisation - Steam sterilisers for pharmaceutical sterilisation products

5.F.2.1 DIN EN 285 (1997-2)

This standard and the steam quality described in it are applicable for large steam sterilisers used in the sterilisation of packed products (instruments etc. and porous goods) in the health industry. It can also be used in the commercial manufacturing of medical devices. This standard cannot be applied for sterilisers used in the sterilisation of pharmaceutical products.

The standard contains comprehensive detailed specifications for the execution of sterilisation using large steam sterilisers. It also describes the technical equipment and procedures for controlling and monitoring the sterilisation process and the steriliser. It provides details of steam composition, dryness (not below 0.9), overheating (must not exceed 25 K in the case of steam that flows freely in the atmosphere), and non-condensable gases. It also specifies a maximum speed of pressure change in the steriliser of 10 bar/min.

The annex of DIN EN 285 provides concrete suggestions for the quality of feed water and steam (for information purposes).

5.F.2.2 DIN 58950 part 7 (April 2003)

This standard is applicable for the sterilisation of pharmaceutical sterilisation products in contact and non-contact sterilisation. This standard specifies three qualities of steam depending on the relevant sterilisation products.

Sterilised product groups

Example assignment of the sterilisation products to different steam qualities (DIN 58950):

Figure 5.F-2 Assignment of sterilisation products to different steam qualities

Sterilisation group I

General clothing and textiles, sterile devices and materials for test laboratories, materials that do not come into contact with the product in sterile areas, parenteral solutions in closed containers, ophthalmics, rinsing solutions for internal applications in closed containers, packed dressing materials.

Steam quality: Sterilising steam

Sterilisation group II

Clean room clothing, primary packaging material for parenterals and ophthalmics, solutions for medicinal products packed under aseptic conditions, including investigational medicinal products, facility components that come into contact with the product, devices, containers, and materials in sterile rooms.

Steam quality: Pharmaceutical pure steam.

This standard distinguishes between two steam qualities (figure 5.F-3):

Figure 5.F-3 This standard distinguishes between two steam qualities:

Heating steam

Steam that is generally used as a heat source and which is produced in steam generators operated using boiler feed water. This steam must not influence the sterilisation products.

Sterilising steam

This is steam with slight impurities that is suitable for a direct effect on the sterilisation products in group I (see sterilisation product groups).

Pharmaceutical pure steam.

This is steam that is suitable for a direct effect on sterilisation products in group II (see sterilisation product groups) due to its particular purity.

The DIN standards also define more detailed specifications regarding humidity, overheating, pressure fluctuations, mechanical filtering, non-condensable gases, etc.

The steam quality to be used in a particular situation depends on the manufacturing process and the product itself. The pharmaceutical manufacturer is responsible for ensuring that steam used in the manufacturing process is suitable for the product.

Applications of pure steam

Pure steam is used for a wide variety of applications in manufacturing operations as well as laboratories. Typical application areas for pure steam are as follows:

  • Heating autoclaves and sterilisers
  • Sterilisation of ultra pure water storage systems and loops
  • Sterilisation of processing systems, containers, vessels, CIP, SIP, etc.
  • Humidification of air conditioners for ventilation of clean rooms, etc.

5.F.3 Pure steam generation

Pure steam generators that are specially designed for this purpose should be used to generate pure steam. Generators are classified according to the natural circulation procedure and downdraft procedure, and pure steam generators with an external heat exchanger. In order to remove non-condensable gases from the steam, the pure steam generator is fitted with an upstream degassing device. Degassing is required in order to comply with the limit values of EN285, e.g. for the sterilisation of textiles and porous products. When using pure steam for other applications, such as sterilisation of an AP storage system and loop, process systems, etc., degassing is not strictly necessary. The requirements for degassing should be clarified during planning in individual cases.

5.F.3.1 Degassing

Degassing is used to remove non-condensable gases from the steam. According to EN285, the proportion of non-condensable gases must not be greater than 3.5 vol. %. As a measurement procedure for the measurement of non-condensable gases, the procedure using a glass burette according to EN285 (laboratory measuring procedure) is very time-consuming, but it is accurate and requires less financial investment.

Automatic procedures for measuring non-condensable gases are also available, such as the Vapocontrol gas detector (made by Mьnchner Medizinmechanik GmbH).

Degassing is either located upstream of the pure steam generator or is integrated within it. The feed water must be degassed to reduce the non-condensable gases.

There are several options for degassing feed water: Degassing using membrane degassing and degassing using degassing containers.

In degassing of feed water using degassing containers, the feed water is pre-heated using a sterile heat exchanger and conveyed to a degassing container, in which the feed water (for the pure steam generator) is maintained at a constant temperature of 90-95 °C. The escaping gas is exhausted from the container via a degassing pipeline.

5.F.3.2 Natural circulation procedure

Figure 5.F-4 Natural circulation procedure (picture taken from Pharmatec)

In the natural circulation procedure, the feed water required for evaporation is supplied via the downpipe integrated in the pipeline heat exchanger. The heat exchangers used for this are equipped with a state-of-the-art double tube plate (to protect against leakage). The downpipe is equipped with an air-cushioned insulation pipe. The insulation of the downpipe creates a temperature gradient from top to bottom. This has the effect that the water is cooler before the heat exchanger than after or above the heat exchanger. The heated pipes of the tube bundle heat exchanger heat the water within them, which causes this to rise based on the change in density in the tube bundle. Cooler water flows from below from the downpipe into the heat exchanger and also flows upwards.

The downpipe in turn acts as a cooler, and water that becomes colder sinks downwards and replaces the water that has risen in the heat exchanger. The insulated downpipe therefore acts as the driving force for the constant circulation. The resulting circulation lends this "natural circulation procedure" its name.

The water that has risen in the heat exchanger is heated to such an extent that steam bubbles form, which continually rise upwards. The steam bubbles in the heat exchanger that is completely flooded with water emerge at the water surface and form a continuous flow of steam. From a certain size of steam flow, when the steam bubbles escape from the water surface, they also take any dissolved water droplets along with them. Larger water droplets fall back down again due to gravity (gravitational separation). The important fact to remember is: The lower the velocity of the steam flow, the smaller the droplets and hence the total mass of water taken up is also smaller. This is important for later separation of droplets and the overall reduction in pyrogens, which should reach at least three log10 levels between the condensate and the raw water of the pure steam generator. The finest water droplets taken up in the pure steam are now separated in coarse separation with baffles (inertia) and then separated in a "bell-bottom separator" (inertia). The pure steam exits the pure steam generator in the centre at the upper pure steam outlet.

5.F.3.3 Downdraft procedure

The feed water enters from below via a pipe that runs through the whole pure steam generator and is conveyed to a point above the heat exchanger, which is located in the upper part of the pure steam generator. The water is heated while being pushed downwards through the heat exchanger into the lower area.

5.F.3.4 Pure steam generator with external heat exchanger

In this type of pure steam generator, the heat exchanger for heating is outside the actual pure steam generator. The feed water is transported into the pure steam generator by a pump. The fill level is kept constant level by a fill level regulation mechanism. The water flows through a pipe connection into the external heat exchanger, where it is heated to the required evaporation temperature. The heated water flows from the heat exchanger back into the pure steam generator, where steam is produced. Because of the low flow rate in the steam generator, small droplets also fall back down into the pure steam generator, while the steam exits at the top following an additional drop collection. Heating the water in the heat exchanger creates a circulation between the heat exchanger and the pure steam generator.

5.F.3.5 Separation systems

An important component of the pure steam generator is the droplet separator. These are initially classified according to their separation rate:

  • Coarse separation
  • Fine separation
Coarse separation

In coarse separation, larger water droplets taken up in the steam are separated. Common procedures for this are as follows:

  • Gravitational separation
    If the flow rate of the steam is low enough, water droplets above a certain size can no longer be taken up in the steam.
  • Baffles
    The flow direction of the steam is abruptly changed using baffles. Based on the higher inertia of the water droplets in relation to the steam, these cannot change direction with the steam. Instead, they crash into the plates and are separated. The flow rate has a significant influence on the degree of separation, but should not be so high that new water droplets are taken up from the separator.
Fine separation

In fine separation, the smaller, finer water droplets taken up in the steam are also separated. The most common procedures used for fine separation are as follows:

  • Bell-bottom separator
    The bell-bottom separator functions, just as the baffles, according to the principle of inertia. Several rapid changes in the direction of the pure steam flow cause even the finest droplets to be separated.
  • Demister
    - No longer used in modern pure steam systems for hygiene reasons.
  • Cyclone separators
    The cyclone separator also functions according to the principle of inertia. In this case, however, the pure steam flow is diverted at a higher speed into a spiral, so that the centripetal force acting on the water droplets presses these against the drainage plates around the outside, where they are separated.

5.F.3.6 Quality-relevant measuring points

The quality-relevant system parameters for pure steam generators are temperature (pressure), conductivity, and TOC content. These variables are monitored online using suitable calibrated measuring devices. The TOC content of the water (condensate) is often determined in the laboratory, since online measurement is not yet specified as a requirement, and online TOC measuring devices are very expensive. Analysis of the condensate for pyrogen content also takes place in the lab ( chapter 5.C.2.2 Sensors).

5.F.4 Pure steam distribution system

An insulated stainless steel piping system is used to distribute the pure steam to the consumers. These piping systems have to meet certain requirements. These include technical requirements, due to the high temperatures (insulation, thermal expansion, etc.), and requirements regarding the appropriate operation of the piping system in pharmaceutical environments, since the steam comes into contact with the product or with surfaces that come into contact with the product (chapter 4.I.3 GMP-conform design of CIP facilities, chapter 5.C.1 Loop).

5.F.4.1 Planning and layout

First, a specification should be compiled, which collects and specifies all user requirements. This should be used as a basis for all further planning steps.

When compiling the user requirements or specifications, the planning steps listed in figure 5.F-5 should be performed so that the acceptance criteria in the user requirements are formulated on a technical and factual basis, and they do not have to be subsequently corrected later in the planning process.

Figure 5.F-5 User requirements for pure steam distribution systems 

User requirements for pure steam distribution systems  

Define and name all project participants and list their tasks and responsibilities.

 

Define the area of use for the pure steam

What will the pure steam be used for?
Sterilisation of ultra pure water distribution systems, containers etc., heating of autoclaves, humidification of air conditioners (e.g. in the clean room area), etc.

Define the required quality of steam

The required steam quality depends on the manufacturing process and the product. It is essential to guarantee that the steam used is suitable for the product. The steam quality should be defined based on the application with the highest requirements. All consumers can then be supplied with this steam. In general, for economic reasons, it is not practical to run two or more steam systems with different levels of quality in parallel.

Define the required quality of feed water

Compare with other possible options for supplying feed water.
Does an ultra pure water system already exist?
Is purified water or WFI really needed as feed water for the required steam, or would demineralised water be sufficient?
If, for example, a loop for purified water already exists, it can be more economical to supply the pure steam generator with purified water than to set up a separate supply with the relevant feed water.
The required quality of the feed water depends on the required quality of steam.

Determine quantity-related requirements

Specify the daily and maximum consumption of all consumers, for rating the capacity of the pure steam generator and the corresponding quantity of feed water required.

Definition of all points of use in the system

Specify the required withdrawal amount, pressure (temperature) and concurrence for each point of use.

Control the points of use

Manual or automated?

Usage characteristics of the points of use

Continual or dynamic withdrawals (erratic)?

Should the hot steam required to heat the pure steam generator be heated by rapid steam generators or by steam generators with continual pressure?

Specify the heating method for the pure steam generator

Hot steam or electric heating? The choice of heating method depends on the available media supply and the required quantity of pure steam. Heating a pure steam generator with hot steam is the most economic method from an energy and efficiency perspective. However, if no hot steam supply is available, the investment costs for this must be taken into account. Alternatively, pure steam generators with electric heating are available, but their capacity is limited.

 

Heating with electricity is considerably more expensive than heating with steam (break-even-point analysis).

Connection to a
WFI still

If possible, this is a very practical method from an energy perspective. The removable quantity of pure steam after the first level of the WFI still is usually approx. 1/6 of the WFI capacity in parallel operation.

Define the required steam pressure and the resulting temperature (figure 5.F-1).

The required steam pressure of the whole system is based on the consumer that requires the highest pressure. The steam pressure for consumers with a lower permitted pressure is reduced. Is it practical to set up different parts of the supply with different steam pressures, or should the pressure be reduced at individual points of use? Careful attention should be paid to pressure-critical components, for example, the containers to be sterilised.

Pressure loss analysis and definition of pipe dimensions

Steam is a compressible medium. This must be taken into account in the pressure loss analysis and pipe dimensioning.

Technical analysis of thermal expansion (compensation for linear extension)

Any linear extension that takes place as a result of heating must be compensated for in order to prevent damage to the system. Angular or horseshoe bends are used, which absorb the linear extension.

Insulation specification

See chapter 5.F.3.3 Downdraft procedure.

Condensate drain

Condensate discarded or with condensate feedback?

If the condensate is drained off and discarded, it must be clarified whether the existing waste water system is suitable for draining condensate (material of the waste water system - clay, plastic pipes, black steel, etc.).

If not, can a condensate cooler be used?

Waste water system

Is it possible to connect the points of use and drainage pipes to the waste water system or does the waste water system have to be extended first?

Which method should be used to operate the pure steam system?

In continuous operation, the system is started once and then remains in operation. In non-continual operation, the pure steam system is started and stopped on a regular basis (e.g. every day). In systems that are not operated continually,careful design of the thermal expansion and condensate drainage mechanisms is particularly important.

Is a steam dryer necessary?

This depends on the construction design of the steam generator.

Sampling cooler

Is cooling water available? If so, in what quantities and at which temperature?

Temperature recording

Does the sterilisation have to be recorded?

Archived on a line recorder or on data transfer media?

Temperature recorders

Are the devices and systems to be sterilised already equipped with temperature recorders at the (coldest) monitoring points?

If temperature recorders are available, it is important to ensure that they also have the correct measuring range for the sterilisation temperatures.

Place of registration

Will the temperature measurements be registered within the control unit of the systems to be sterilised (e.g. ultra pure water distribution system, containers, autoclaves, etc.) or in the control of the purified water system (21CFR11)?

Safety technology

Insulation

Are all components of the pure steam system that can get hot protected against unwanted contact by insulation or contact protection ?

Emergency off buttons

Are emergency off buttons for the pure steam generator desirable or required at different points in the loop (e.g. at points of use)?

Safety regulations

How should the machine be operated to comply with safety regulations (instructions, standards, legal requirements, etc.)?

Pressure levels

The pressure levels of the components to be used must be taken into account.

Temperature resistance

The temperature tolerances of the elastomer materials must be considered.

Dimensioning and determination of nominal widths

In contrast to the calculation of pressure loss in ultra pure water distribution systems, in which the flow cannot be compressed, the flow in steam systems is compressible. When gases and steam flow through pipes, this is an expandable flow. In this type of flow, the pressure decreases in the direction of flow due to friction loss along the pipes (in the same way as incompressible liquids). Because the medium is compressible (gas, steam etc.), these pressure changes also cause changes in temperature (independently of heat loss), density, and velocity.

Unlike an incompressible flow, the pressure drop along the pipe is not linear and the flow rate is not constant (see figure 5.F-6).

Figure 5.F-6 Incompressible and compressible flow

The pressure and velocity profile along the pipes depends on the type of expansion and on friction. A distinction is made between two types of piping: insulated and non-insulated pipes. Steam lines should always be insulated for safety reasons, as well as to prevent the steam from cooling and condensate from forming. If we assume that the insulation could completely prevent any heat from being exchanged from the steam to the environment, this would be an adiabatic flow. In non-insulated pipes, heat exchange takes place between the medium and the environment. The flowing medium adjusts to the ambient temperature (e.g. in underground gas lines). In this case, the flow can be described as very similar to isothermic flow. Both types of flow, isothermic and adiabatic, are theoretical borderline cases, since in a real pipeline flow, a certain amount of heat exchange always takes place, and the temperature does not always remain constant.

The calculation of pressure loss in steam lines is highly complex and is performed in practice using computer programs or nomograms. For more detailed information, consult specialist literature on the subject. It should also be mentioned that in practice, steam distribution systems are designed with a flow rate of 20-40 m/s.

5.F.4.2 Condensate drain

Regardless of pipe insulation, some condensation will always form in a steam distribution system, primarily in the heat-up phase following downtime. This condensate has to be drained off at particular points in the pure steam system. Due to the fact that considerably more condensate is formed during the heat-up phase, steam systems have to be started up slowly and carefully. If the condensate drainage system were designed for the condensation formed during the heat-up phase, this would be far too large for normal operation. In order to catch the condensate that forms during heating, condensate supports should be used that have the same nominal width as the nominal width of a steam line (see figure 5.F-7). They should be long enough to collect the condensate that forms during the heat-up phase. When the piping has reached a particular temperature, the condensate drain must then be able to drain off the remaining condensate. It is also important to take into account that when the system is started up, the full operating pressure has not yet been reached, which means that less condensate can be pushed through the condensate drain due to the steam. The quantities of condensate involved, both for the heat-up phase and in normal operation, must be determined in advance (during planning) and the condensate drain must be designed accordingly.

Condensate drains should be installed:

  • At every point of use
  • Before a pressure regulation valve or pressure limiter
  • On all vertical sections of the pipe
  • Depending on the environmental conditions (insulation thickness, flow rate, ambient temperature etc.), after certain lengths of the pipeline
  • At the end of each section of pipe (to enable flow through the section of pipe)

Some important points to consider in the design of the condensate drains:

  • Condensate drains should not be installed in sequence (one after the other).
  • Steam lines with different pressures should not be drained into the same condensate drain without the necessary precautionary measures.
  • Hot condensate should not be drained into a cold condensation drain. The hot condensate, under a lower pressure, undergoes re-evaporation. If this comes into contact with condensate that is too cold, it would condense abruptly.
  • The condensate pipe must be designed so that it exerts the minimum of pressure on the condensate drains.
    If pressure in the condensate pipes cannot be avoided, this must be taken into account in the design of the condensate drains (type and size). Even if this is the case, the pressure in the condensate pipes should always be lower than the pressure in the steam lines.
  • Condensate pipes must be laid so that they drain completely, and must empty by themselves.

It is also important to consider whether the drained condensate will be discarded, or re-used and hence used in the generation of new pure steam. If the condensate is re-used, the condensate that collects at the individual condensate drains is collected in condensate collection pipes and fed back to the pure steam generator. However, it is an essential prerequisite that the condensate is not contaminated in any way by the steam from which it originates. For quality assurance reasons, condensate feedback is not often used in pure steam distribution systems in the pharmaceutical industry. When condensate is re-used, it is usually only for condensate from line drainage.

Systems for draining condensate
  • Condensate drainage with bypass
    The condensate collects in a dead end (e.g. a T-section). The condensation increases, and when it reaches the branch, is drained via the condensate drain. The correct functioning of the condensate drain can be monitored through an observation window. If the condensate drain fails, the system can continue to operate via the bypass (170 HV 003 and 170 HV 006 are closed, 170 HV 002 is opened). Condensate drainage via the bypass is only a short-term emergency solution, due to the loss of steam. It is also possible to equip a bypass with a second condensate drain. The condensate can then be drained through the bypass for longer.

Figure 5.F-7 Condensate drain with bypass

Figure 5.F-8 Condensate drain without bypass

  • Condensate drain without bypass
    Functions in the same way as the version with a bypass, only without the bypass (170 HV 002). For cost reasons, this variant is often used in pure steam systems in which pure steam is not required continually (24 h/day 365 days a year), and hence the system can be shut down for service and maintenance.
Condensate drain systems

There are various different types of condensate drain, which function according to various principles:

  • Mechanical float condensate drains
    • Spherical float trap
    • Spherical float trap condensate drains
  • Thermal condensate drains
    • Thermal capsule condensate drain
    • Thermal bimetal condensate drain
    • Accumulation condensate drain
  • Thermodynamic condensate drain
    • Thermodynamic condensate drain
    • Impulse condensate drain
  • Rigid condensate drain
  • Active condensate drain
    • Condensate drain with pump action

The mechanism of action of two condensate drain systems commonly used in pure steam systems is described below. This is an example and does not mean that these are the only two possibilities.

Spherical float trap

The spherical float trap drain is a mechanical condensate drain (see DIN 3680). A float is mounted in the condensate drain housing, which operates the flow valve of the drain. If the condensate drain housing fills with condensate, the float moves upwards due to the lifting force acting on the float, thereby opening the outlet valve. If the condensate has been pushed out of the condensate drain due to the pressure in the steam line, the float closes in accordance with the condensate level in the outlet valve. Inflowing steam cannot escape through the condensate drain (figure 5.F-9).

Figure 5.F-9 Advantages and disadvantages of spherical float trap condensate drains

Advantages

Disadvantages

  • Condensate is drained immediately at saturated steam temperature without accumulating.
  • Reliably sealed due to water trap.
  • High capacity at low pressures.
  • Condensate drainage is independent of load and pressure fluctuations.
  • Very good ventilation properties if suitable ventilation elements are used.
  • Spherical float trap condensate drains are susceptible to frost.
  • They should not be used if there is a danger of water hammer.

Since the outlet valve only opens when a suitable amount of condensate is in the condensate drain housing, a thermal deaerator is integrated in the system. This ensures that when the pure steam distribution system is started up, any air that remains in the pipes can escape through the condensate drain.

If hot steam enters the condensate trap, the thermal deaeration mechanism closes and the steam cannot escape through the condensate drain.

Thermal capsule condensate drain

Thermal capsule condensate drains are thermal condensate drains (see DIN 3680). These belong to the thermo-elastic category of condensate drains which work with pressurised dispensers that are partially filled with liquid. The pressurised dispenser operates the closing mechanism depending on the pressure and temperature. Since the saturation temperature of steam and condensate is the same, the condensate drain is designed so that it only opens the closing mechanism once the condensate has been cooled to a certain level. In practice, condensate drains have a specified cooling of between 5 and 30 Kelvins, depending on the construction and adjustment. Due to their design, they follow the course of the pressure-specific boiling point (see figure 5.F-10).

Figure 5.F-10 Advantages and disadvantages of thermal capsule
condensate drains

Advantages

Disadvantages

  • Safe mechanism of action due to exact correspondence to the saturation curve.
  • Drainage of the condensate with undefined super-cooling below the saturate steam temperature.
  • Robust and compact.
  • Reduced sensitivity to water hammer and frost.
  • Excellent ventilation characteristics.
  • Large capacity range with small dimensions and weights.
  • Due to the need to super-cool the condensate, it is important to be careful when using overheated steam.

Since the air in the steam lines are at a lower temperature (and pressure) than the steam, the condensate drains can also be used for deaeration. Because the pressure and the temperature of air-steam mixtures are below the boiling point and the corresponding pressure of the steam (Dalton's law), air-steam mixtures are also drained through the condensate drain.

As the condensate must be cooled to a certain level before it is drained, sufficient volume should be available upstream of the condensate drain to allow the condensate to collect here for cooling.

5.F.4.3 Insulation

Steam lines become very hot due to the heat of the steam flowing through them. In the specification for insulation, the investment and maintenance costs for the insulation have to be weighed up against the costs resulting from heat loss. The design of the insulation is influenced by such factors as the steam temperature (and the resulting pipe temperature, the flow rate, the diameter of the pipe, and environment conditions such as temperature and air movements (e.g. wind, draughts, etc.). The fact that insulation has to be taken into account in the planning and installation phases of the pure steam distribution system is more important than the thickness of the insulation itself. This ensures that sufficient space is allowed around the actual pipe, including any mountings, for the insulation and its installation. The heat loss from areas that cannot be insulated is several times greater than that caused because the insulation is not quite thick enough.

The insulation specification not only has to take into account the insulation thickness, but must also define the insulation material and the insulating jacket. The insulating materials and the insulation thickness should be specified in cooperation with a suitable specialist company. When defining the insulation jacket, it is important to define clear specifications. These should be fulfilled according to the specific environment in which the piping is laid (services area, suspended ceiling, false floor, clean room area, etc.).

The following variants are frequently used in practice:

  • Aluminium-clad (aluminium foil)
  • Aluminium jacket (aluminium sheet, riveted)
  • Stainless steel jacket (stainless steel sheet, riveted)
  • Pipe-in-pipe system (no rivets and screws)

For use in clean room areas, the type of jacket is important. The surfaces must satisfy the requirements of good manufacturing practice (can be cleaned, no edges and corners where dirt can accumulate, durability against cleaning agents, etc.). Cost plays an important role in the selection of the jacket, as the investment costs vary greatly between the individual variants (aluminium-clad < aluminium sheet < stainless steel sheet < pipe-in-pipe system). The pipe-in-pipe system is so expensive that for such high demands, it is also worth considering laying insulated steam lines in enclosed media channels.

5.F.4.4 Pressure reducing valve

A pressure-reducing valve is used to reduce the pressure in pipes. The valve is a spring-loaded proportional valve that controls the pressure in the direction of flow and regulates it to a predefined value. Pressure-reducing valves are available in a range of different types (with membranes, bellows, pistons). In a membrane-controlled pressure-reducing valve, for example, the force exerted on the membrane due to the reduced pressure is at equilibrium with the preset spring force. If the pressure before the pressure-reducing valve now increases, more steam flows through the valve (opening between the valve surrounding and the sealing surface of the piston). This causes the pressure at the membrane to rise, and a greater force is exerted on the membrane.

The piston rod is moved against the spring force until the valve opening has reduced sufficiently so that the pressure acting on the membrane and the spring force are once again at equilibrium. If the pressure before the valve is reduced, the valve opens far enough until the force equilibrium is restored. In an unpressurised state, this type of valve is fully open.

Pressure-reducing valves for pure steam should fulfil the requirements of suitable implementation for pharmaceutical use as far as possible, although it is often necessary to make compromises, since the selection is limited to the valves available on the market. Many manufacturers are now offering products that fulfil many aspects of these requirements.

5.F.4.5 Safety valve

Safety valves protect steam systems (steam generators and distribution systems) against excess pressure. Safety valves must comply with the applicable safety regulations. Each individual valve is subjected to a 100 % quality control. Safety valves are set by the manufacturer to a preset pressure and open 100% immediately when the set pressure is reached. They do not close until the pressure is 10% below the set pressure. When a safety valve opens, large amounts of steam must be allowed to escape the system rapidly for immediate pressure reduction. For this reason, the nominal outlet width of a safety valve is always greater than the inlet width.

So that the steam escaping from the valve does not simply escape into the environment when the valve is activated and cause injury to people, the steam outlet must be controlled and steam guided via a suitable pipe. In this pipe, the escaping steam must only be subjected to a minimum resistance. It should therefore be short in length and laid with as few bends as possible. The steam must exit from the pipe in a location at which no people or property are can be out at risk, injured, or damaged (steam is therefore frequently extracted via the roof). Safety valves and outlet pipes must be dehydrated so that any water within them does not cause resistance to the escaping steam.

5.F.4.6 Pipe connections

Pipes must also be connected in a manner suitable for pharmaceutical manufacturing. Steam systems are exposed to powerful thermal influences as a result of the dramatic differences in temperature, for example  between downtime (approx. 20 °C) and operation (e.g. approx. 150 °C).

Due to the linear extension that takes place, the tension within the system also varies greatly. For this reason, clamp connections and screw fittings should never be used in steam systems. All pipes should be connected using flange connections. In screw fittings, there is a danger that these may come undone due to the temperature change. Clamp connections can be loosened by hand and should never be used in steam systems for safety reasons.

5.F.4.7 Sampling cooler

In order to analyse the quality characteristics of pure steam, the condensate of the pure steam is analysed (see chapter 5.F.2 Quality requirements for pure steam). Sampling coolers are used to cool the steam to a level that allows non-hazardous, safe and reproducible sampling, without endangering the person taking the samples. A small heat exchanger is used to cool the steam. In principle, for example, a small tube bundle heat exchanger can also be used, although this would involve significant investment costs. In practice, for cost reasons, ready-made sampling coolers are used that have been specifically designed for this purpose. The construction of these sampling coolers must also comply with the requirements of good manufacturing practice.

When using a sample cooler of any variety, it is important to ensure that suitable cooling media are available at the installation location. To ensure reproducible results, the procedure for taking samples should be regulated by a suitable SOP.

Summary

Steam is used as a heat transfer medium in technological installations due its high level of stored heat energy (enthalpy, see figure 5.F-1). It is used, for example, for heating autoclaves and sterilisers, for sterilising ultra-pure water storage systems and loops, process systems, containers, vessels, CIP, SIP, etc. Unlike ultra pure water, no requirements for pure steam are specified by the FDA or in the pharmacopoeias (e.g. Ph.Eur., USP), and no limit values have been defined for the chemical and physical properties of pure steam.

Specifications regarding quality requirements for pure steam are provided in DIN EN 285 "Sterilisation - Steam sterilisers - Large sterilisers" and DIN 58950 "Sterilisation - Steam sterilisers for pharmaceutical goods".

Pure steam generators that are specially designed for this purpose should be used to generate pure steam. Generators are classified according to the natural circulation procedure and downdraft procedure and pure steam generators with an external heat exchanger. In order to remove non-condensable gases from the steam, the pure steam generator is fitted with an upstream degassing device. Before a pure steam system is realised, it should be planned in detail and all user requirements incorporated into a specification.

The design of a distribution system must take into account any thermal expansion. To avoid combustion and minimise energy loss, sufficient insulation is essential. When dimensioning the pipes it is important to note that steam, unlike ultra pure water, is a compressible medium; this means that several points in particular must be noted.

An effective and correctly functioning condensate drain (pressure impulses) is an essential feature for a pure steam system. The pure steam system must be equipped with suitable technical components for the procedure (valves, condensate drains, measuring technology, safety valves, etc.).

Due to the high temperatures, it is often necessary to meet a compromise between the usual GMP requirements and the steam components available on the market. For safety reasons, all steam systems must be secured against excess pressure with the use of suitable approved components (excess pressure valve, bursting disk, etc.).



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