Flow Control, Flow Control April 2019

Figure 3 A general service control valve such as this Fisher Model V270 is designed for line pressures of up to 1480 psi General service control valves General service applications do not require noise attenuation cavitation protection or tight control therefore general service valves do not have splined shafts or attenuators but are still equipped for dynamic applications Severe service applications are those in which excessive noise or cavitation is expected A general service trunnion mounted and full bore pipeline ball valve as shown in Figure 3 can be used in both control and isolation applications It has the mechanically retained seals necessary for throttling without the risk of seal failure Taking a high pressure drop across the valve while throttling at low degrees of travel can cause nonmechanically retained seals to be sucked out of the carrier resulting in damage to the seal and preventing the valve from being shut off Other features include live loaded packing and a robust drive train designed to absorb energy properly at mid travel As the control valve throttles it absorbs energy creating side loading on the valve shaft The live loaded packing guides the valve shaft under these conditions Mechanically retained seals prevent damage to the trim during pressure drops often experienced when flow is throttled down significantly When selecting a general service control valve look for Design compliance with both B1634 and API 6D Mechanically retained seal inserts Live loaded packing A general service control valve can be used in almost any application that does not require noise reduction and extreme precision However a double D or pinned ball shaft connection does not provide the highest performance because general service applications generally do not have stringent throttling requirements Severe service valves Typical severe service applications require high performance actuation This could be needed for fast stroking speeds or higher cycles Proper O rings actuator accessories and other valve components and features need to be considered when selecting valves for severe service To greatly reduce lost motion a severe service trunnion mounted and full bore pipeline ball valve as shown in Figure 4 has splined ball to shaft and shaftto actuator connections Cavitation can be a problem in severe service applications When pipeline fluid flows through a fixed restriction like a control valve the pressure drops Depending on the severity of the pressure drop and the fluid properties associated with the process this can cause cavitation Cavitation occurs when the pressure at the vena contracta falls below the vapor pressure of the process and then recovers downstream Cavitation can lead to extensive damage to valve bodies and trim In gas service noise and vibration can be generated by high pressure drops across the control valve therefore dealing with these sources of cavitation and noise is essential in preventing damage to the valve in severe service applications Pipeline ball valves can be supplied with various types of anti cavitation and noise protection trims to stage the pressure drops and prevent cavitation This allows the valve to not take the entire pressure drop at once Aerodome style trim can be used to keep the noise level down to a reasonable level These trims can be customized for different startup cases and for full capacity to fit the desired application When selecting a severe service ball valve look for Anti cavitation trim Splined shaft connections Fast stroking capabilities Adaptable product platform for special constructions Pipeline applications Ball valves can be used in the following applications Within a gate station As a worker monitor valve To provide over pressure protection As a pump station pressure control valve Within a compressor station To provide anti surge protection Figure 4 A severe service anti surge valve such as this Fisher Model V260A has a rugged actuator and other features for fast stroke long life and high performance in demanding applications 18 April 2019 Flow Control