An Introduction to Diaphragm Seals

 

Converting fluid or gas pressure into force or motion

Hydraulic and pneumatic systems that do this invariably use  some form of dynamic seal.

The energy conversion in such systems is usually accomplished by a cylinder—an enclosed, stationary barrel containing a moveable piston. Liquid or gas introduced under pressure into the cylinder exerts  force on the piston head, creating motion that can pull, push, position, open, close or regulate a mechanical device or assembly.To keep the pressurized fluid from leaking past the moving piston, a dynamic (motion- tolerant) seal of some kind is required. Several types of dynamic seals are commonly used, with the selection depending on the specific requirements of the application.Fabric reinforced molded diaphragms are flexible, circular seals, molded under high temperature and pressure, with concentric 3D contours. The contours flex to permit axial motion of the central area with respect to the outer circumference. 

Reinforced diaphragms are produced  from tough, flexible elastomer (which serves as a fluid barrier), bonded to one  or both sides of a high tensile strength fabric that supplies the mechanical reinforcement needed to withstand high pressures. They are widely used instead of interference seals and flat diaphragms wherever a relatively long piston stroke is needed, and where dynamic and breakaway friction would adversely affect the speed, accuracy or repeatability of an operation, and/or where blow by leakage cannot be tolerated. 

Fabric reinforced molded 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 

RPP diaphragms are routinely used in such diverse applications as process control equipment, water management, gas regulation, precision medical equipment and in industrial systems such as   pumps, tensioning devices, vibration apparatus and metering devices. Since they can be custom designed quickly and inexpensively to meet a wide variety of conditions and requirements, RPP diaphragms frequently represent invaluable solutions to equipment design problems.

Molded diaphragms are composed of tough, flexible, carefully engineered laminates. In general, the laminates are made out of elastomer and fabric. Metal or plastic inserts are sometimes molded in to serve as a piston or rigid backing for a valve seat. A layer of PTFE may also be molded on to improve resistance to harsh chemicals and abrasives. 

The elastomer is one or two pressure-tight layers of tough elastomeric (synthetic rubber) 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 to one or both sides of a fabric base. The fabric is  a high tensile strength base layer of highly engineered woven fabric, designed to withstand the stresses induced by high pressures. Medium tensile strength base fabrics are also available that have been specially designed for ultimate sensitivity and low hysteresis.

Advantages

  1. Virtually no friction
  2. No break-away friction
  3. Fast response to small changes in pressure – accurate positioning
  4. Minimum wear  - minimum maintenance
  5. No blow-by leakage
  6. No lubrication needed
  7. Zero spring rate

 Disdvantages

  1. Generally higer costs than interference seals
  2. Longer installation time, 
    in some instances

 

Document Download files

 

Diaphragm Formulas (661KB PDF)