Like any other well-established technology, valves have a wide variety of designs and options. At times, this variety can make it difficult to compare valves. In the spirit of National Mentoring Month, we thought we’d introduce you to some valve basics, from terminology to use. While there are many types of valves, in this article we’ll focus on diaphragm valves as characterized in the ASME BPE Standard, particularly radial-diaphragm valves and weirless diaphragm valves.
Section SG 126.96.36.199 of the 2014 edition of the ASME BPE lists the types of diaphragm valves as:
- Weir Diaphragm Valve, Weir Diaphragm Tank Bottom Valve
- Radial Diaphragm Valve, Radial Diaphragm Tank Bottom Valve
- Weirless Diaphragm Valve
- Linear Control Valve
- Regulator Valve
What classifies all of these as diaphragm valves is that they have a dynamic seal that shuts off or controls flow through the valve, and they have a static seal that prevents process fluids from escaping into the atmosphere. In the image below, you can see the two different seals.
2-inch Radial-Diaphragm Tank Valve assembly
As you can see, in a radial-diaphragm valve, both of these seals are combined into a single unit called a diaphragm. In applications where cleanability and sterility are critical, this type of sealing is preferred over multi-piece seals. While these two seals vary in shape between different diaphragm valve types, both are necessary for any diaphragm valve to function properly. If either of these seals fails, it can lead to contamination of a process batch.
You might be wondering what all the rest of the parts shown in the valve assembly are and what they do. Let’s take a look at the primary components of a typical valve, illustrated below.
The valve body is the main component of the valve and is either welded, clamped, or bolted into a vessel or process line depending on what the end user requires. Here are a couple standard valve bodies.
The main features of a radial-diaphragm valve body are the seat, shoulder, inlet, outlet, and chamber.
ASEPCO Radial-Diaphragm Tank Valve
The main features of a weirless diaphragm valve body are the seat, shoulder, inlet, and outlet.
ASEPCO Weirless Diaphragm Valve
The seat of any valve is the primary working surface and one of the most critical components. This is the surface that the dynamic seal of the diaphragm closes against to shut off flow through the valve. The other critical surface is the shoulder or seal of the valve. This is where, typically, the combination of actuator, diaphragm, and valve body form a static seal against the outside atmosphere. The remaining features of the valve are necessary, but less critical.
The inlet and outlet are the points where fluid enters or leaves the valve. In the case of a radial-diaphragm valve, there is also a chamber. The chamber’s primary function is to create a space where outlet(s) can be added to the valve body. Additionally, it provides space where the diaphragm can move in order to open and close the valve.
It should be noted that after a valve body is installed in a system, unless it is abused, it can work perfectly for years. Due to the static nature of the valve body itself, with all the work being performed by the diaphragm and actuator, it does not tend to see any wear over time, like the diaphragm or actuator. This brings us to the most important component of the valve assembly: the diaphragm.
As we noted earlier, the diaphragm provides the sealing for the valve and is made up of a dynamic and static seal. The following images show these seals for radial and weirless valves.
Because the diaphragm is made of elastomers or polymers and is subject to mechanical stresses throughout its lifecycle, it is considered the wear part of the valve. This means it is the part of the valve assembly that is most often subject to replacement. The diaphragm supplies the barrier between your product and the outside world, so its maintenance and regular replacement is one of the most critical items in your processing system. When it comes to choosing the diaphragm, there are many options to consider.
While there are many diaphragm materials to choose from, our requirements in the bioprocessing industry limit us to those that include requirements like USP Class VI. The most common materials are Silicone, EPDM, and PTFE. Each of these materials has advantages and disadvantages. When choosing your diaphragm material, there are many influences to consider so that you can ensure the best performance/price for your application, such as:
- Process fluid exposure and compatibility
- Process temperatures exposure and compatibility
- Exposure times to each fluid time and temperature
- Preventive maintenance (PM) schedules
- Future applications
It important to consider these items in advance, not just because it allows you to choose the most suitable diaphragm for your application, but more importantly, it allows you to avoid potential problems in validation. For example, although it happens rarely, there is the possibility that someone specifies a diaphragm material that is perfectly suitable for testing during Factory Acceptance Testing, but fails in actual process conditions. This can happen for any number of reasons, but if you confirm the details of your application with your valve supplier early in your design process, you can easily avoid a scenario like this.
If you spend enough time in this industry, you come across valve failures. They can fail for many different reasons and in many different ways. In many cases, the failure is related to the diaphragm (directly or indirectly) and can be resolved in a number of ways. I won’t go into those details here, but you can learn more by using the links below:
While the diaphragm is considered the wear part of the valve, the actuator is the real workhorse. You’ll find that you have to replace the diaphragm in your valves regularly but that the actuator serves as long, or almost as long, as the valve body. There are two types of actuation that are normally used in the biopharmaceutical industry: manual and pneumatic.
Even though actuators have a lot of components, most are internal and you’ll probably never see them unless you are in maintenance, where they are occasionally rebuilt. The parts of the actuator that you do see and use regularly are pretty simple:
- Shaft: The shaft is where the diaphragm is mounted to the actuator. This is often threaded or otherwise attached directly to the shaft. In some cases, there may be an intermediate component, such as a compressor that helps support the diaphragm in use. The shaft is also often used as a visual indicator and protrudes from the bottom of the actuator when the valve is open.
- Housing: The housing is the largest part of the actuator and encloses all of the working components of the actuator as well as supporting the diaphragm.
- Knob/handle (manual actuator): In modern terms, this would be called the human interface device, or as we used to call it the thing you turn to open and close the valve.
- Air inlet (pneumatic actuator): This inlet is where the air source is attached to open the valve. When the air is removed or turned off, a spring inside the housing closes the valve. (In some cases this is reversed—the air is used to close the valve and the spring is used to open it.)
- Breather vent (pneumatic actuator): This vent is where air is pushed out on the spring side as air is introduced into the air inlet and where air is pulled in as the air source is closed/removed. (Note: Plugging the breather vent with a solid cap prevents the actuator from functioning.)
As you can see, there are just a few parts you need to interact with in order to use an actuator. In addition to most of the components being sealed inside the housing of the actuator, the actuator is also not regularly exposed to process fluids. Coupled with the fact that most actuators in the biopharmaceutical industry are actuated very infrequently (compared to other industries), this means that they typically are maintenance free for years. That said, there are some do’s and don’ts that can be helpful in maintaining your actuators:
- Perform visual and operational inspections during PM cycles
- Confirm pneumatic actuator seals are not leaking during PM
- Perform regular maintenance
- If cleaning by submersion, make sure the actuator is fully sealed and designed for this type of cleaning
- Don’t hit actuators with forklifts
- Don’t drop actuators from second or third floor mezzanines
- Don’t hurl actuators across the room into the parts washer
As odd as it may seem, all of these things have happened. It is interesting to note that, outside of the extraordinary circumstances in the “don’ts” list, actuators that are regularly maintained rarely malfunction.
That wraps it up for valve basics. You should now be able to identify the basic components of a diaphragm valve, the major features of each component and the function of those components.