The mysteries of diaphragm seals revealed

Unlike most other types of seal, diaphragms are not designed around industry standards, and there is a lack of knowledge about what actually goes into the design beyond the performance advantages

Moulded diaphragms often provide the best possible solution to a sealing problem - and frequently they are the only solution.

They are widely used wherever accurate, reliable response to hydraulic or pneumatic pressure changes is needed.

For example, when process control valve needs to respond to slight changes in the downstream process valve actuators friction can often prevent a precise and instant response.

To overcome this and ensure the actuator is sensitive enough to respond to minute changes in feedback signals, a diaphragm seal is used.

No other type of seal will offer the advantage they can provide in this critical function.

Similarly the regulation of the pilot pressures and signal pressures in any control loop depends on regulators that employ a precise force/balance equilibrium and institutes corrective action.

In both of these applications the unique advantages of a diaphragm seal cannot be sustained without compromising the performance of the entire system.

It is because of this that process control engineers use diaphragm seals as the heart of their design.

They know that a diaphragm seal has no breakaway friction, is sensitive to slight pressure changes and has no blow-by leakage.

But unlike most other types of seal, diaphragms are not designed around industry standards.

As a result, there is a lack of knowledge about what actually goes into the design beyond the performance advantages.

Diaphragms are produced in a wide range of shapes, different types of construction and many different types of materials.

It is easy to answer the question as to why a diaphragm seals should be custom designed to give maximum performance.

This involves its shape, construction and materials.

Diaphragm Seals depends on stroke requirements, effective area, flange design and assembly considerations.

Diaphragm Construction depends on diaphragm shape, hardware designs and pressure application.

Diaphragm Materials are dependent on fluids or gases being used, operating temperature and pressures.

The first considerations in defining the shape of the diaphragm are to establish the effective area and stroke requirements.

The effective area will define the piston and cylinder diameters.

The gap between the piston and cylinder - the convolution width - is dimensioned to minimise the rolling stress in the diaphragm.

The stroke requirement will define the height of the diaphragm.

The ability to mould the diaphragm into the as-installed shape being dependent on the ration of the height to the cylinder bore diameter.

After the diameter and height have been established then flange design is considered.

Flat flanges will accommodate bolted together hardware and beaded assist in assembly and reduction in component size.

For the type of construction it is always desirable to use double-coated design.

With elastomer on both sides of the fabric the diaphragm is protected from abrasion and delamination.

However, when the height/diameter ratio is too great single-coated construction must be used with elastomer on the pressure side of the diaphragm only.

Double coating will tolerate pressure from both sides of the diaphragm and single coating from one direction only.

Single coating, however, allows for the maximum of stroke capability for a given diameter.

Having determined the shape and type of construction the correct materials must be selected.

Moulded diaphragms are composed of tough, flexible, carefully engineered laminates.

In general, the laminates have two basic elements.

1 - A high-tensile-strength base layer of specially woven fabric, designed to withstand the stresses introduced by high pressures.

2 - One or two pressure-tight layers of tough elastomeric material selected (or specially compounded) for long life, under a specific set of operating conditions (fluid type, temperatures, etc).

Depending on the application, the elastomer is bonded either to one or to both sides of the fabric base.

The elastomer is selected on the basis of compatibility with the application fluids and gases and the operating temperatures.

The elastomer layer must be extremely thin (for maximum flexibility), very tough (to withstand repeated flexing over a long lifetime).

Able to handle anticipated operating temperatures, and resistant to whatever chemicals or fluids it may be contacting.

A number of standard elastomers are currently available which usually meet most application requirements.

In unusual cases, however, special materials can be produced.

The fabric used in moulded diaphragms employ a specially developed weave pattern with a unique property.

In one direction, it stretches easily, allowing for the radial flexing of the diaphragm convolution.

In the other (axial) direction, it provides high resistance to elongation and furnishes the tensile strength that enables the seal to withstand high differential pressures and transmit force to the piston head.

Ideally diaphragm seal design aims at the best balance between shape, construction and materials.

If the stroke/cylinder bore ration can be kept to less than 1:2 then the diaphragm can be made in the as-installed shape with double-coated construction for optimum durability.

Whilst temperatures in the range of -40 to 100 degrees C can use elastomers that minimise cost.

With differential pressures below 10 bar than fabric reinforcements can be utilised that will optimise fabric-elastomers adhesion and cycle life.

When designs are tailored to obtain the best combination of the shape, construction and materials then opportunities for cost improvement arise.

While diaphragm seals are the heart of the device they are used in, they are generally combined with other components to complete the internal assembly mechanism.

Often components such as pistons and valve seats can be integrated into one diaphragm components that improves performance whilst significantly reducing costs.

All of these design considerations provide insight into the mystery of why a particular diaphragm design is utilised.

It becomes apparent that there is much more involved with proper diaphragm design than unique performance advantages.

The design decisions must consider the desired performance advantages, hardware design, overall cost and ease of manufacture.

Because of all these considerations, it is important to custom design for each application.

Attempting to adapt existing designs can overlook significant design considerations.

Every application has specific requirements that dedicate tailoring a diaphragm to meet them.

For example, valve actuators often utilise cast pistons and must be easily serviced in the field.

Double-coating a diaphragm protects it against abrasion from the cast surfaces and moulding in the as-installed shape, facilitates assembly.

Quarter turn valves have a long non-linear stroke for a given diameter.

A long stroke-coated diaphragm with an extra wide convolution width to accommodate the non-linear travel is the ideal solution.

Whilst it is important to identify the specific requirements of an application, it is also important to collaborate with the user in development of a specific diaphragm design to arrive at the optimum for each application.

In addition the collaboration process between diaphragm producer and user provides an opportunity to become aware of the various design concepts rather than being confronted with the mystery of how and why a particular diaphragm seal design came to be used.

Ceetak have over 25 years of experience on providing engineering sealing solutions.

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