THE MISUNDERSTOOD CHECK VALVE - DFT Valves

1 AS SEEN IN THE SUMMER 2006 ISSUE Back to BASICS: TOO OFTEN. THIS VALVE . IS UNJUSTLY. BLAMED WHEN. PROBLEMS. OCCUR, BUT. THE REAL. CULPRIT. IS THE THE MISUNDERSTOOD . APPLICATION. BY MIKE JOHNSON CHECK VALVE . C MISUNDERSTOOD Valves ever heck Valves are probably the most they continue to work even if the plant facility loses air, electricity, or the human being that might manually cycle them. invented. If you mention CHECK Valves to As with other types of Valves , CHECK Valves are found in a full range of sizes, most plant personnel, the typical materials, and end connections. The line sizes range from 1/8 inch or smaller to 50. response is they don't work. In fact, inches and larger. They are made of bronze, cast iron, plastics, carbon steel, and those personnel may well have taken various grades of stainless steel, alloys such as Hastelloy, Inconel, Monel, and tita- out the internals or repiped the system nium.

2 End connections include threaded, socket weld, butt weld, flanged, Victaulic, to avoid CHECK Valves . In other words, wafer, and insert type. these Valves are the least popular VALVE CHECK Valves are found everywhere, including the home. If you have a sump in use today. pump in the basement, a CHECK VALVE is probably in the discharge line of the pump. This article will explore the basics Outside the home they are found in industries such as automotive, desalination, avi- of CHECK Valves , how they work, what ation, commercial construction, water and waste, chemical, colleges and universi- types there are, how to select and ties, food and beverage, geothermal, hospitals, mining, oil and gas, pharmaceutical, install them, how to solve their prob- power, pulp and paper, refining, sanitary, marine, steel, tire, and ultrapure water.

3 Lems, and, finally, why they are not Like other Valves , CHECK Valves are used with a variety of media: liquids, air, always the cause of the problem. other gases, steam, condensate, and in some cases, liquids with fines or slurries. Simply put, a CHECK VALVE allows Applications include pump and compressor discharge, header lines, vacuum break- flow in one direction and automati- ers, steam lines, condensate lines, chemical feed pumps, cooling towers, loading cally prevents back flow (reverse racks, nitrogen purge lines, boilers, HVAC systems, utilities, pressure pumps, sump flow) when fluid in the line reverses pumps, wash-down stations, and injection lines. direction. They are one of the few self-automated Valves that do not How They Operate require assistance to open and close.

4 CHECK Valves are flow sensitive and rely on the line fluid to open and close. The While some can be fitted with exter- internal disc allows flow to pass forward, which opens the VALVE . The disc begins nally weighted and dampened devices closing the VALVE as forward flow decreases or is reversed, depending on the design. for special circumstances, the major- The function or purpose of a CHECK VALVE is to prevent reverse flow. Construction is ity do not have any outside assis- normally simple with only a few components such as the body, seat, disc, and cover. tance, such as that found with on/off Depending on design, there may be other items such as a stem, hinge pin, disc arm, or other Valves . Unlike other Valves , spring, ball, elastomers, and bearings. 28 | VALVE M A G A Z I N E 2006 VALVE Manufacturers Association.

5 Reprinted with permission. B A C K T O B A S I C S : C H E C K V A L V E. Internal sealing of the CHECK VALVE disc and seat relies on reverse line Lift Ball pressure as opposed to the mechanical force used for on/off Valves . Because of this, allowable seat leakage rates are greater for CHECK Valves than with on/off Valves . MSS SP-61 Pressure Testing of Steel Valves , published by the Manu- facturers Standardization Society, is one standard used by manufacturers to perform seat and shell closure tests for CHECK Valves (as well as other Valves ). Factors affecting CHECK VALVE seat leak- The ball CHECK VALVE is similar to the lift CHECK , but uses a ball instead of a guided disc. age include reverse pressure, medium, and what the seat material is made of, disc movement occurs inside the VALVE , the flow path and the flow through the such as metal or elastomer.

6 Metal since the disc is always in the flow VALVE does not affect the performance of sealing surfaces generally will allow path. This results in wear, potential for the wedge whether that flow is low, some leakage while elastomers such as failure, and a higher pressure drop medium, or high. Buna-N and Viton provide bubble-tight than calculated. Various types of CHECK Valves are shutoff (zero leakage). Because of this, Whenever a metal part rubs against available. Some of the more popular elastomers should be considered for another metal part, wear is a result, types are included below. All these can air/gas media and low-pressure sealing. which leads to eventual failure of the be used for clean media. As with other Important considerations when using component.

7 A component failure can types of Valves , specialty CHECK Valves elastomers for such Valves are service result in the VALVE not performing its can be found for special applications. temperature and compatibility of the function, which in the case of a CHECK While no one type of VALVE is good for elastomer with the medium. VALVE , is to prevent reverse flow. In all applications, each has its advan- tages. Taking time to contact the manu- Regardless of type or style of VALVE , the facturer to assist in selection can help longest trouble-free service will come from you find the best fit, especially if you are incurring problems with whatever type Valves sized for the application, not the line of CHECK VALVE is presently installed. size, whereby the disc is stable against the internal stop in the open position or fully Ball Checks closed.

8 Ball CHECK Valves use a ball inside the body to control the movement of flow. So what is the ideal CHECK VALVE ? extreme cases, failure could result in The ball is free to rotate, resulting in Regardless of type or style of VALVE , the the component or components escaping even wear and a wiping action between longest trouble-free service will come into the line, causing failure or non- the ball and seat. This feature makes ball from Valves sized for the application, performance of other Valves or equip- checks useful for viscous media. Ball not the line size, whereby the disc is sta- ment in the line. checks are typically found in smaller ble against the internal stop in the open Typically, pressure drop is calculated sizes of 2 inches and less. Some designs position or fully closed.

9 When these con- based on the CHECK VALVE being 100% include a spring to assist in closing and ditions are met, no fluttering of the disc open as with on/off Valves . However, if for use in 90-degree styles installed in will occur, resulting in premature fail- the flow is not sufficient to achieve full vertical lines. Depending on the body ure. Unfortunately, most CHECK Valves opening, but instead, the CHECK VALVE is design, pressure drops with ball types can are selected in the same way on/off only partially open, the pressure drop be higher than with other types of CHECK Valves are selected based on line size will be greater than calculated since the Valves . Ball checks are available in vari- and the desire for the largest Cv avail- flow passage is restricted by the disc ous end connections including threaded able.

10 This ignores the fact that, unlike being in the flow path. In this situation, and socket weld. Some body designs per- on/off Valves , the flow conditions deter- a large rated Cv actually becomes detri- mit in-line repair/inspection. mine the internal performance of the mental to the CHECK VALVE (unlike with CHECK VALVE since its disc is always in on/off Valves ), resulting in fluttering of Dual Plate the flow stream. the disc and eventual failure. Such is Dual plate, or double-door, CHECK Valves As mentioned earlier, unlike on/off not the case with some other Valves . consist of two plates hinged in the cen- Valves , CHECK VALVE internals are flow With a gate VALVE , for example, if the ter and spring loaded to assist in clos- sensitive. If there is not enough flow, VALVE is fully open, the wedge is out of ing.