Diaphragm Seal Selection
Selecting a reinforced, molded diaphragm seal that will perform adequately in a specific application is a relatively simple matter. Selecting and specifying the seal that will provide maximum performance and the longest possible service life with the least possible investment calls for a degree of expertise gained only through years of experience in diaphragm seal design and application. Since no two applications have precisely the same conditions, requirements and problems, RPP engineers and application specialists apply their special knowledge in developing the optimum seal specification for each individual job. Selecting and specifying the molded, fabric-reinforced diaphragm that will perform best in a particular application can be a complex task. A surprising number of parameters and physical considerations are involved, and the final design often depends as much on the experience and understanding of the design engineer as on standardized engineering considerations. As a result, it is usually preferable, and more cost-effective, to custom design the seal to fit the job rather than to attempt to use a standard, off-the-shelf product. Nevertheless, certain guidelines can be used in selecting the general type, configuration and, in some cases, the materials best suited to a proposed dynamic seal.

Seal Configuration
Selection of the overall form of the diaphragm is determined, generally, by the need to conform to existing or planned hardware and/or special considerations imposed by the mechanical operation to be performed.  Such considerations include:

• Power stroke direction:  uni-directional or bi-directional (i.e., is actuating pressure applied from one side or alternately from both sides of the piston)

• Stroke length: If the projected piston motion is relatively long, a rolling diaphragm is generally required.  For shorter stroke lengths, offset or planar types can be used.

• Flange type: Beaded, for quick, easy installation or flat for higher pressure capabilities and for less expensive hardware.

• Effective area: If an application requires a constant actuating pressure across the total piston stroke, a rolling diaphragm is required since it is the only configuration providing a constant effective area throughout the stroke cycle

Operating Pressures
It is the base layer of fabric that enables the diaphragm to withstand the stresses imposed by the actuating pressure. The anticipated maximum differential pressure, therefore, will determine the type and strength of fabric to be used. In addition, if back pressure is developed at the bottom of the piston stroke (i.e., if the cylinder is not vented), this will affect the type of coating specified.

Operating Temperatures
The temperature extremes anticipated in an application play a major role in the selection of both elastomer and fabric.

Working Fluids
The chemical nature of the fluid (or fluids), which will be contacting the diaphragm naturally, plays a key role in the anticipated service life of the diaphragm. Care in the selection of the elastomer is needed to provide maximum resistance to corrosive or harsh media.

Diaphragm Selection Tool