Design Considerations for Steam Generator Specification

Steam has various applications in the pharma industry, mainly used for heat-related purposes. Some processes require raw steam to be acceptable, while others require the highest purity steam levels, commonly known as Pure or Clean Steam. Because the application is critical and raw or plant steam can affect the process or product, pure steam is the best possible alternative.  

Pure Steam has a high purity level and is designed for critical application in the pharma industry. Its main objective is to provide high temperature, while at the same time preventing contamination with microorganisms & foreign bodies when using steam for critical applications.

What is Pure Steam?

Pure Steam is generated from water with a standard equal to drinking water, USP specified, or Water For Injection - WFI. Since the feed water is free from contamination, foreign particles or other impurities, the steam obtained is also free from additives, contamination, microorganisms, and foreign particles. 

This contrasts with plant steam, which is generated from tap or raw water. Since water is not treated for contamination and other impurities, the steam obtained from this water also contains impurities.

Due to the purity levels of pure steam, they are used in critical applications where the steam comes in direct contact with the pharma product or critical items. Examples of some processes where clean steam is used includes the following.

· Sterilization - steam sterilization of items to kill microorganisms and prevent contamination. 

· Cleaning - cleaning of various items used in the pharma industry. 

· Production requirements - clean steam produces high temperatures for production processes , such as product mixing.

The generation of pure steam requires dedicated equipment, other than the pant boiler. This is due to the difference in the type of feed water.

Pure Steam Generator

A pure steam generator is equipment used to produce clean steam. The feed water is heated until it converts its phase from water to gaseous, i.e., steam, like the usual boiler. This steam then flows through the relevant port, pipe, or discharge for usage in appropriate applications.

During steam generation, removing non-condensable gases and entrained droplets from the steam is necessary because it will degrade the steam, reduce its effectiveness and cause contamination in the application process. Removing of such elements is often achieved by the mechanical design at different location.

Components of Pure Steam Generator

There are several components in a pure steam generator that have their definite function, and they include the following:

Feeding System

The feeding system consists of multiple sub-components. It is used to facilitate the water entering the pure steam generator with the desired flow characteristics. It consists of

· Appropriate piping for connecting the water infeed to the steam generator

· Pump for feeding the water with the required volume of water. Since the feed water from the plant does not have the required flow characteristics, flow must increase to suit the steam generator’s requirements.  

· A filter system to purify the infeed water according to the specification of the steam generator. Some manufacturing applications use purified water for infeed, so in that case, filters are not required

Evaporator and Steam generation

It is the main component and is responsible for converting water into steam. It consists of a shell & heat exchanger. A heat source provides the desired temperature. Depending upon the design, the heat source could be plant steam or electricity.

In plant steam design, steam from the boiler heats the heat exchanger. On the other hand, in electricity-based design, heaters are used to increase the temperature.

With the effect of high temperature, the water is converted into steam. Since the feed water is purified, the generated steam is free from contamination, microorganisms, and foreign bodies.

Condensate Separation

At this stage, liquid water or condensate remains in the steam, hindering its effectiveness. In the separation section, liquid is separated from steam using a mechanical design.

Steam Storage

When the generator generates the steam, it is not directly discharged to the distribution system. Instead, it is stored in the tank.

Steam is released into the distribution network from the tank.

Steam Discharge

This section transfers the steam from the tank section through appropriate piping to the distribution system.

Instrumentation System

Like other pharma processes, it is necessary to monitor the steam generators, its process and output  by measuring different variables. Monitoring helps ensure safe & effective working, and also ensures that steam generated is according to the required specification.

Some standard instrumentation includes the following.

· Infeed water flow - to measure the volume of water flowing into the steam generator

· Infeed steam pressure - to measure the pressure of the plant steam, if the heating mechanism is plant steam

· Input voltages for electrical heaters - to measure the voltages for heaters, if the heating source is electrical heaters 

· Outfeed steam pressure - to measure the pressure of the pure steam generated

· Temperature - to measure the temperature at different parts, such as the evaporator and the steam

· Water Level - to measure the water level of the infeed in the infeed water tank

· Conductivity - to measure the conductivity of the infeed water

· Data logger – some steam generators also use data loggers to log the steam quality parameters against time for various regulatory requirements

Control System

The Control system operates the steam generator and executes its functions. It consists of electrical & automation components, such as Programmable Logic Controller, Human Machine Interface, Sensors, and other components depending upon the equipment design.

Safety System

Safety systems in steam generators are integral to safe operations. Because steam under a pressurized stage  has a lot of energy. If appropriate safety systems are not built, steam can explode different mechanical structures,such as storage tanks & piping, and can also cause human burns.

Different types of safety features are deployed in the seam generator, including the following.

Over Pressure - This safety prevents overpressure at the discharge stage, when steam is ready for pharma applications. It ensures the steam's pressure does not exceed the storage tank's limit. There are two types of overpressure safety - the safety valve & pressure switch.

· Safety Valve is a mechanical device with a specific release rating, which is adjustable and is set according to the tank's capacity. When steam pressure crosses the tank's capacity ( and also of the valve), the valve automatically opens and releases the steam, until the tank's pressure reduces to the acceptable range.

· Pressure Switch safety is an electronic safety that sends an electronic signal to the controller when the tank's capacity is reached. The signal is then programmed to execute various safety processes, such as stopping the process and/or alarm indication on the steam generator.

Over Temperature - this safety prevents over temperature at various points in the steam generator. It consists of a temperature sensor ( thermocouple or RTD), connected to the central controller. When there is an over-temperature condition, defined by user input, the controller indicates an alarm and automatically shuts down the process.

Steam Quality - As mentioned earlier in this article, it is used to check steam quality through conductivity continuously.

Design Considerations for Steam Generator Specification

Let's discuss the steam generator specifications relevant to the pharma industry.

Infeed water Specification

As mentioned earlier, the clean steam is free from impurities. For this purpose, the infeed water is purified using appropriate filtration mechanisms. The specification of infeed water must be equivalent to drinking water or WFI.

Infeed water Volume

Another specification of infeed water is its supply pressure. Because the steam generator's output depends on the volume of infeed water. The output will be reduced if the infeed water supply drops below the designed water supply.                

If the plant water supply has enough volume to match the generator's supply, it can be directly connected to the steam generator. On the other hand, if the plant water supply cannot match the generator's requirement, a pump must be integrated into the generator to generate the desired water volume.

Testing points

Testing points are sampling points that allow you to take steam samples during the generation process. It helps to analyze the steam effectiveness for various pharma-related applications, such as sterilization.

The steam generator should include testing points for taking out samples. Their location must be selected for easy sample taking, and should represent the actual steam condition.

Degasser

It is a mechanical device used to separate non-condensables from the steam. The steam generator must have a degasser to remove non-condensables from the final product.

Material of Construction

The construction material for the steam generator must be SS316L. This is because the steam is corrosive which can corrode the metallic parts. To prevent corrosion, SS316L, which is highly resistant to corrosion, must be used.

Another advantage is that since the output stream comes in direct contact with the critical items, SS316L is also recommended for these applications.

Non-metallic components, such as pipes, must be made of PTFE because they can prevent contamination.

Surface finish

Surface finish defines surface smoothness and is critical in resistance to the buildup of microorganisms on the metal's surface. It is measured in terms of surface roughness, i.e., Ra.

For a clean steam generator, the surface finish must be < 0.5 μm