Diaphragm Seal Applications
Diaphragm Sea Selection
Selecting and specifying the diaphragm seal that will perform best in a particular application can be a complex task. A surprising number of parameters and physical considerations are involved, and a successful design often depends on a complex multi-variable optimization relying on the experience and understanding of the engineer rather than a straightforward step by step engineering method. 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 diaphragm seal.
The typical molded diaphragm will fall into one of four categories: Rolling “Top Hat”; Convoluted; Offset-Convoluted; and Double Acting. Rolling diaphragms are used when a constant effective area is needed in an application with a high stroke/diameter ratio. Convoluted diaphragms offer maximum cycle life and are pre-molded into the installed shape for ease of installation. An Offset-convoluted diaphragm is the choice for longer stroke lengths and ease of installation. Actuating pressure can be applied from both directions in Double-acting diaphragms, which are usually double-coated to withstand harsh media.
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:
*Stroke Requirement – The diaphragm height relates to the overall height and diameter of the piston and the convolution height and width. The piston travel on each side of the clamping flange determines the height of the diaphragm. 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.
Beaded, for quick, easy installation or flat for higher pressure capabilities and for less expensive hardware.
Flange Design - Flange retention design is a function of cost and application requirements.
Beaded Flange - The bead simplifies assembly of the diaphragm to the hardware. The seal is enhanced by the bead, which spreads under pressure in a properly sized hardware groove.
Opposing Beaded Flange – With double-coated applications RPP can produce diaphragms with opposing beads and even offset-opposing beads where the top bead is not in alignment with the lower bead.
Flat Flange – Used for larger diaphragms and all high pressure applications. Flat flange allows for less expensive hardware.
The Effective Area/Diameter is the center line between the cylinder bore and the piston diameter. It represents the pressurized area of the diaphragm. 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.
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 the 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 Pressure requirements drive fabric choice.
In the axial direction, fabric provides high resistence to elongration and furnishes the tensile strength that anables the seal to withstand high differential pressures.
In the radial direction fabric stretches to all a diaphragm to roll with a constant effective area.
The temperature extremes anticipated in an application play a major role in the selection of both the elastomer and fabric.
The chemical & physical nature of the fluid (or fluids) and gases, 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.
The elastomer and operating media must be compatible. Dounload our Elastomer and Media Compatibility chart or contact RPP directly for help with your specific requirements.