DIAPHRAGM TERMINOLOGY
In specifying diaphragm configuration, certain terminology is involved.
- Effective Area:
- The effective area of a diaphragm, essential to performance is instrumental in the regulation of the fluid in the system. This effective area is subject to many variations.
- If there is any movement of the diaphragm, the effective area is not measured in terms of its total diameter. Instead, it is measured in terms of an imaginary diameter—one that is dependent upon the travel involved, shape, center plate dimensions and flange design.
- Effective area multiplied by pressure equals resultant force. Mathematical treatments of this property are available. At best, these approaches are estimates. Actual test measurements are the most accurate means for determining effective area. Final performance is measured around this critical portion of the diaphragm.
- Travel or Stroke:
- Travel is the distance through which the diaphragm moves in operation. Travel is directly related to the volume of fluid moved or regulated by the diaphragm. With dished diaphragms, long travel distances can cause wide variations in effective area; with convoluted types, this variation is not great.
- Convolution:
- In a convoluted diaphragm, the effective area is composed of a molded curved section concentric with the circumference. Convolutions allow great diaphragm travel, approximately twice the depth of a non-convoluted diaphragm.
- In designing convoluted sections of diaphragms, the engineer should verify that the convolutions are practical to manufacture. A good rule of thumb is not to exceed a one-to-one ratio between convolution depth and width.
- Center Plate:
- Sometimes called pistons or center discs, center plates are used to reinforce the diaphragm. They may be used in pairs or in combinations of several types, ranging from flat with slightly rounded corners to curved with fully rounded corners. Most center plates are made of metal or other rigid materials. Diaphragm travel is dependent upon size and shape of center plates.
- Flange:
- This is the clamping area where the diaphragm is held and sealed. Like center plates, flange shapes may vary from flat to curved to beveled. In addition to the role they play in effective area and travel, they perform an important function in diaphragm assembly. To eliminate the danger of cutting or tearing the edges of the diaphragm, a correct flange type must be chosen. Problems with diaphragm pull-out usually originate with an improperly sized or faulty flange.
- Spring:
- Springs control the forces acting on the diaphragm. In many diaphragm systems, springs are used to supply return movements to the starting position. They also furnish a constant force against which the diaphragm works. They should be constant-rate types, not subject to wide changes in their properties.
- Pull Rod:
- A pull rod is firmly attached to the center plate of the diaphragm. Through it, diaphragm motion is transferred to actuate other parts in the system. The pull rod must be affixed to the center plate in a way that will allow the diaphragm to travel with minimum eccentricity.
- Housing:
- The entire diaphragm system is contained in the housing. This part may be stamped, die-cast or machined. It should be nonporous to resist fluid corrosion and strong enough to withstand operating conditions.
