Defining severe service valves

Severe service valves can be found in non-return, isolation and control functions, although it is recognized that Severe Service Control Valves (SSCVs) do have reasonable industry agreement on what can define severe service. Severe Service Isolation Valves (SSIVs) do not have nearly as clear agreement or understanding.

A determination of whether severe service exists for a control valve can be applied through thresholds expressed in Table 1*. It should be cautioned that the potential for severe service is an indication rather than a proof and further examination and analysis should be performed.

When any of the above conditions violate the thresholds in Table 1*, the application should be considered as severe service and the selection of the SSCV should be made by suppliers who specialize in control applications using valves designed with severe service trims and features. Additional process information should also be reviewed.

Severe service conditions always apply to the following:

Isolation valves perform a different function than control valves. During much of their installed life they are static, like the pipe flange they are installed within. Typically the valve datasheet provides us with adequate information for valve selection for this state. As with control valves for severe service applications, more and deeper information and consideration is required to select a Severe Service Isolation Valve (SSIV) Industry adheres to Codes like ASME B16.34, B31.1 and B31.3 to protect for this static state, but these do not provide much guidance for when the valve is in dynamic situation.

An important element in selecting SSIVs is the consideration of what can occur while the SSIV is transitioning from its static normal state to the other(Open to Closed, Closed to Open); when it is in dynamic conditions. During this dynamic state, conditions can be vastly different than when static. For example, closing a valve against a normal flow rate from its fully opened position will accelerate the fluid up and until it is stopped by the closed valve, while at the same time decrease its pressure (Bernoulli’s Principle) to a point where it may flash or choke. It is important to have a firm idea of the number of cycles from one starting position to another and the normal position for each application.

There are several typical scenarios: low frequency normally closed (NC), low frequency normally open (NO), frequent NC and NO and equal NC/NO. The frequency of cycling isn’t in itself the challenge, its knowing what happens on each cycle to affect the valve’s health and ability to retain the minimal performance required by the application. If a valve has an allowable leakage rate (see FCI 70.2-2013) like most metal seated valves have, then the fluid can become erosive if the energy level (differential pressure) is high enough to propel the fluid past the closure element, and severe degradation of the sealing elements can occur. This might seem to indicate that a higher number of closed cycles are likely to produce valve health issues than less cycles, but each application has its unique needs, and a valve that only cycles once per year may be as difficult to perform as one cycling once every six hours.

Isolation valve datasheets also often fail to provide a goal or target for installed performance during the valve’s useful life other than the risky assumption that the required valve closure test from API 598, ISO 5208 or MSS-SP-61 can translate a similar quality into the valve’s actual duty and performance in service. Therefore more attention should be focused upon arriving at a reasonable Severe Service Isolation Valve (SSIV) definition, as it is in our opinion, the more difficult task. A severe service isolation valve data sheet lists the key elements that should be identified in order to properly select the type, materials and options of the SSIV, including the information required to determine the effects of the dynamic status like cycle rate and media affects.

SSIVs should