In fluid flow control systems it is frequently required to reduce fluid pressures by several hundreds of pounds per square inch in order to maintain flow control. Pressure drops of this magnitude are commonly accompanied by cavitation and generation of audible noise. Generally, cavitation will occur in a liquid system when the pressure is reduced below the vapor pressure of the liquid, at which time vapor bubbles form in the liquid. When, as in the case of a control valve, throttling is followed by pressure recovery, these vapor bubbles collapse or implode, generating shock waves in the liquid. These shock waves commonly result in severe erosion, or "cavitation damage," to valve parts when conventional plug and orifice valves are employed. Such damage, of course, leads to premature valve failure, having serious economic consequence.
Heretofore it has been common to treat the problems of cavitation, noise generation and metal erosion in fluid flow control valves in several ways.
Reference may be made for instance to the following U.S. Patents of interest: U.S. Pat. Nos. 3,637,188; 3,715,098; 3,485,474; 3,776,278; 3,880,399; 4,634,095; 2,832,371; 3,482,604; 5,018,703; 5,113,908; and 5,415,202.
Generally, such references illustrate that the use of multi-stage valve trim is in common practice in high pressure drop, pressure reducing valve situations when there is a possibility of cavitation. Many of these references provide a multi-stage valve trim with a lengthy fluid flow path of tortuous or labyrinthine configuration within the internal elements of a valve and wherein most of the reference trims use small size flow passages and tight clearances between the parts through which the flow must pass. In situations where the process fluid is clean, the small passages work well and provide the desired reduction in pressure while reducing the risks of any cavitation occurring.
However, in certain situations it is required to utilize a pressure reducing fluid control valve where the process fluid is not clean and otherwise may contain particulates in the flow stream of relatively large sizes, and at times larger in size than the small flow passages in the pressure reducing valve. Using currently available pressure reducing valves in such severe circumstances can lead to the small flow passages becoming plugged by the particulate material in the flow stream. Such plugged passages reduce the flow capacity of the valve and lead to valve trim damage from the large size particulates flowing in the fluid stream. Furthermore, if the valve trim with small sized flow passages is replaced by a trim stage having larger size flow passages because of the plugging conditions, this reduces the amount of pressure drop to be supplied by the valve and also can lead to cavitation problems.
It is therefore desired to provide a fluid control valve with multiple stages of pressure reduction and which can reliably operate under severe flow conditions where the fluid may contain entrained particulates of large size.