Most experts agree that SSVs are identified by applications and that these applications are challenging to the valve’s ability to provide a minimum acceptable level of performance over a minimum acceptable duration.
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.
Non-return (check) valves for severe service applications should be treated as control valves and sized so that their operation is consistent with the flow-rates of the process rather than the pipe size they are typically selected for.
Control valves take energy out of a piping system; isolation valves contain the energy and non-return valves delay and reduce the energy from its full effects on the isolation and control valves. All valve design functions require basic information, but those valves destined for severe service require a deeper understanding of all of the factors that affect their in-service performance.
Figures 1 and 2 are a snapshot of the dynamic fluid state through a control valve showing the energy lost during the pressure drop. Figures 3 and 4 show various combinations of mechanisms that can effectively absorb that energy and protect the valve and downstream components from damage.
Identification of severe service conditions for control valves may be determined by performing sizing calculations using IEC 60534-2-1 or ISA 75.01.01 with the following information:
A determination of whether severe service exists for a control valve can be applied through thresholds expressed in Table 1. It should be noted that the potential for severe services is only an indication and not proof and therefore further examination should be performed. However, it is apparent that the further beyond the condition threshold, the more severe the service.
Excellent tools currently exist to reduce risk and time to perform the extensive calculations that are required to test for the condition thresholds; one of the best is Flowserve’s Performance!, which uses a Valve Selection Guide to significantly reduce sizing and selection inaccuracies, and provides clear and abundant data to assist in defining the conditions within and around the control valve.
When any of the 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:
Control valves that do not violate any of the conditions in Table 1 or are not identified in the above list of severe service applications can be identified as general purpose control valves.
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 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) 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 SSIV definition, as it is in our opinion, the more difficult task.
A SSIV 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.
In addition, severe service conditions always apply to the following isolation applications:
By using suppliers who specialize in SSVs, applying a series of steps in qualifying them and separating general purpose from the more challenging applications, greater and longer success for a facility’s valve population is achievable.
Diagrams and certain thresholds provided by Flowserve.
Written by Ross Waters