All About Diaphragm Valves
Basic engineering devices such as valves contribute to almost every complex system. These mechanical/electromechanical devices regulate media flow and come in dozens of unique varieties; to see all the different types of valves, visit our understanding valves article. The diaphragm valve will be the focus of this article, and we will explore what they are, how they work, and the different types available. Through this investigation, this article should help decide if diaphragm valves could be useful, and how to choose the correct version for a given application.
What are diaphragm valves?
Diaphragm valves are bi-directional, on-off throttle valves. They are used to control fluid flow by regulating the area with which media can enter and exit the valve, effectively changing its speed and velocity. They are so-called “diaphragm” valves because a thin, flexible membrane is used to control the opening and closing of the valve. They can be made with metal such as stainless steel, plastic, and even single-use materials. Diaphragm valves are like pinch valves but they use a linear compressor which pushes the thin diaphragm into contact with the valve body. They offer discrete and variable pressure control and can be both manually and automatically actuated. Diaphragm valves are clean, leak-proof, easy to maintain, safe, and effective valves that are best suited for moderate pressure & temperature applications which require stop/start fluid control.


How do diaphragm valves work?


Diaphragm valves are simple in construction and operation. As shown in Figure 2, a valve actuator is in contact with the inner membrane (or “diaphragm”) of the valve in its open position. When the user desires the valve to be shut, the actuator is pressed and/or turned and the membrane is pushed into the edge of the solid damn, closing the valve. Certain diaphragm valves can also have half open/restricted positions where the membrane is partially closed, allowing a throttled flow through the valve. In the next section, we will examine the broad categories of diaphragm valves and where they are most useful.


It’s a diaphragm valve but not as we know it




Bürkert’s Robolux diaphragm valve addresses wastage and dead volume with an innovative 3-way design that eliminates dead legs and any need for heavy valve block assemblies. Process Industry Forum spoke to Ian Webster, Field Segment Manager for Hygienic Processing at Bürkert, to find out how Robolux has changed the diaphragm valve as we know it.


Robolux provides a three-way diaphragm valve


A diaphragm valve is the only truly hygienic valve. There are other valves that are listed as being hygienic, which are used in breweries and dairies, but in the ultra-pure biotechnology and pharmaceutical industries they exclusively use diaphragm operated control valves.


This is because diaphragm valves are easy to drain; they’re two-way, so have two connections; there are no crevices inside; they provide a smooth flow through them; and you can polish the insides, so they’re very sterile.


The problem with a standard pneumatic diaphragm valve is that it’s just a single seat valve with an inlet, an outlet and an actuator on it. The diaphragm opens and closes to let flow through the valve. With a lot of valves used in industry, you can have a two-way valve and a three-way valve, so you have a third port inside the valve and the actuator closes one port and opens another.


You can’t do that with a traditional diaphragm valve because it’s only a two-way valve. Bürkert introduced the Robolux valve, which employs two pistons in one actuator to create a three-way type diaphragm valve.


Compact and lightweight 4-valve seat blocks


A conventional diaphragm control valve has a weir across the middle that seals the flow from one side to the other. The Robolux valve has two weirs and a gap in between the two, which is like a third port. The actuator allows you to open one weir, so the flow comes in and then goes out through the middle outlet.


You can close that weir and then open the other weir and get flow from the other way and that goes out of the outlets. Conversely, if you have flow coming up from the middle, you can divert it one way or the other. It then becomes a divert or mixing valve. There is no other pneumatic diaphragm control valve that can do that.


The beauty of the Robolux is that we’ve developed a way in which you can put an actuator on either side of the valve. If you take this valve with two weirs on one side and then machine the same on the other side you then have four valves in one small block with two actuators on it. A 4-valve seat block using a conventional pneumatic diaphragm valve is quite a big lump of stainless steel. We can get the same functionality with a much smaller lump of stainless steel.




Robolux delivers 50% less dead volume


If you have a diaphragm control valve in a certain orientation and you open it, it will drain itself. When you start to combine these in a block, so that you have multiple seats, you can never get it to drain – there’s always an internal dead volume.


That can be a problem, certainly for some parts of the Pharmaceutical industry, where the product they’re manufacturing is very high value. That little volume of the product could be worth several thousand pounds. What we’ve done with the Robolux valve is make that block a lot smaller. Also, because we’ve got this double weir valve, the internal dead volume inside the valve is reduced by 50% or more.


The Robolux valve reduces that internal waste and so reduces the cost of lost product. It also makes it possible to make the valve block more complicated and get more valves into it because each valve in itself is a 3-way valve rather than just a 2-way valve. With Robolux, you can get more diaphragm valves in the same space of valve seats than a conventional one.


Method Of Control


Diaphragm valves use a flexible diaphragm connected to a compressor by a stud which is molded into the diaphragm. Instead of pinching the liner closed to provide shut-off, the diaphragm is pushed into contact with the bottom of the valve body to provide shut-off. Manual diaphragm valve is ideal for flow control by offering a variable and precise opening for controlling pressure drop through the valve. The handwheel is turned until the desired amount of media is flowing through the system. For start and stop applications, the handwheel is turned until the compressor either pushes the diaphragm against the bottom of the valve body to stop flow or lifts off the bottom until flow is able to pass through.


Diaphragm Valve Function


The diaphragm is connected to a compressor by a stud molded into the diaphragm. To start or increase flow the compressor is moved up by the valve stem. To stop or slow flow, the compressor is lowered and the diaphragm is pressed against the bottom of the valve. Diaphragm valves are excellent for controlling the flow of fluids containing suspended solids and offer the flexibility of being installed in any position. Weir-type diaphragm valves are better at throttling than straight though diaphragm valves because the design's large shutoff area along the seat gives it the characteristics of a quick-opening valve. The diaphragm acts as the gasket of the valve to seal against leaks between the body and bonnet cap.


Types


Diaphragm valves are available in two basic forms: weir type and straight through types. The basic construction of both valves is similar except for the body and diaphragm.


The weir-type design is the most popular type of diaphragm valve and it is best for general use applications or for tough corrosive and abrasive services. They are best used to control small flows. The body of the weir-type has a raised lip that the diaphragm comes into contact with. Weir-type valves use a smaller diaphragm because the material does not have to stretch as far. The material can be heavier so the valve can be used for high-pressure and vacuum services. The weir design is composed of a two-piece compressor component. To create a relatively small opening through the center of the valve, the first increment of stem travel raises an inner compressor component that causes only the central part of the diaphragm to lift instead of the entire diaphragm lifting off the weir when the valve is opened. Once the inner compressor is opened, the outer compressor piece is lifted along with the inner compressor and the additional throttling is similar to the throttling function in other valves. Weir-type bodies have bonnet assemblies recommended for handling dangerous liquid or gas because if the diaphragm fails the hazardous materials will not be released into the surrounding system. They are also recommended for food-processing applications because the body is self-draining.


The straight through type can be used in situations where the flow direction changes within the system. The body of this design has a flat bottom that is parallel to the flow stream. This allows the flow to move uninhibited through the valve with no major obstructions. A flexible material is required for the diaphragm so that the mechanism can reach the bottom of the valve body; this can shorten the life span of the diaphragm. They are excellent for use with sludge, slurries and other viscous fluids but they are not well suited for high temperature fluids.


Media


The flow transfer and control category covers a wide spectrum of products designed to facilitate, control, maintain, meter, or read the flow of material through hoses, pipes or tubing. The material in question can be liquid, gaseous, or semi-solid (colloids and slurries). The following families fall within this category: valves, valve actuators and positioners, dispensing valves, pumps, flow sensing, level sensing, density and specific gravity sensing, viscosity sensing, and miscellaneous related products.


Diaphragm valves are especially suited for handling corrosive fluids, fibrous slurries, radioactive fluids, or other fluids that must remain free from contamination. Since the diaphragm does not come into contact with the media the valve can be used with sticky or viscous fluids, which can get stuck or clog other types of valve mechanisms.


Valve Components


Diaphragm valves have a very basic body construction.


Body


The stem of a diaphragm valve does not rotate.


The stem of this valve can be non-indicating, which means that the handwheel rotates a stem bushing to engage the stem threads. This moves the stem and attached compressor up and down. The diaphragm is attached to the compressor. Non-indicating bodies can use sealed bonnets with a seal bushing.


The stem can also be indicating. This design operates the same as the non-indicating design but the stem is longer and extends up through the handwheel. Indicating bodies can use a sealed bonnet with a seal bushing and O-ring.


The bonnet of a valve is the cover of the top of the valve; it contains the nonwetted portion of the valve, the compressor, and the handwheel mechanism. The bonnet is then bolted to the valve body. Diaphragm valve bonnets are quick opening and lever operated. This type of bonnet is interchangeable with the standard bonnet on conventional weir-type bodies. The lever opens and closes the diaphragm with a 90° turn. Diaphragm valves can also be used in vacuum services using a bonnet construction up to 4 inches in size. For larger valves, a sealed, evacuated bonnet should be used.


A compressor is required to operate the diaphragm in the valve. The compressor is located above the diaphragm and below the handwheel stem. The compressor is rounded and shaped much like the body's flow passage.