The term “severe service” has been used to develop new valve markets and even create popular new valve types, such as severe service ball valves. That term also has likely been misapplied as much as it has been correctly used.
If you are a process control operator in a plant, you might consider every energy-containing pipe run and valve to be severe service, since the results of a valve failure could be costly or hazardous to your health. Although virtually every valve contains potentially dangerous energy or controls potentially hazardous ﬂuid, the degrees of danger vary. These service criteria are used to determine the valve’s design or the robustness of a valve to safely handle a particular service application.
Often a standard off-the-shelf, “general purpose” vanilla valve will sufﬁce in a particular service and perhaps last for decades without replacement. When a few operating or ﬂuid characteristics are changed, the vanilla valve may need to have a bit of chocolate syrup and a cherry on top (a ﬁt-for-purpose valve) to operate effectively. And when the combination of service criteria is harsh, a banana-split (severe service valve) is called for.
Some ﬂow control conditions/service applications that are often considered severe include the following:
- Autoclave let-down
- Boiler feedwater
- Choke valves
- Coal gasiﬁcation
- Compressor anti-surge
- Fluids with high outgassing potential
- HP separator drains
- Minimum ﬂow recycle
- Molten salt
- Toxic/lethal service
Severe service valves are offered in several standard designs, including non-return (check valves), isolation and control types. There is generally agreement in the valve industry as to the deﬁnition of a severe service control valve; however, isolation and check valves do not yet have that degree of differentiation, so a proper severe service deﬁnition will be welcome for those valve types. Non-return valves for severe service applications should be treated as control valves and sized so their operation is consistent with the ﬂow rates of the process, rather than the pipe size they are typically sized for. This resizing often goes against the perceived logic that the check valve size should match the pipe run size.
A MOST DIFFICULT JOB
Severe service isolation valves may have the most difﬁcult job in a piping system. They are designed to move from a static state (either fully open or fully closed) to a dynamic state, usually in a matter of seconds. The effects of this near-instant energy release combined with the requisite pressure/temperature and other service-related issues often create a severe environment in which the valve must operate.
Although all these valves, especially the isolation valves, have been closure tested in accordance with a variety of production testing standards, such as the American Petroleum Institute (API) 598, Manufacturers Standardization Society (MSS) SP61, or the International Organization for Standardization (ISO) 5208, these tests are hydrostatic and not dynamic in nature, so the exact emulation of in-service ﬂow control is not achieved. This situation makes gathering and analysis of all the service conditions especially important in the selection of the proper severe service isolation valve.
So just what does “severe service” mean? To clarify the deﬁnition of the phrase, the Manufacturers Standardization Society (MSS) is in the process of creating a standard practice (SP) that will help deﬁne severe service in a logical, scientiﬁc way.
SCOPE OF NEW MSS STANDARD PRACTICE
The new MSS SP is appropriately titled Severe Service Valves, and its stated purpose is: “To provide a method to categorize valves by the severity of the service conditions they will encounter in an application. The categories are to be used as a guide for specifying appropriate requirements for special design features, materials of construction, non-destructive testing, quality inspections, cleaning, painting/coating and testing relative to the service conditions the valve will encounter.”
The purpose further provides that “the deﬁnition and common categorization of valves by their severity of service assists in the proper speciﬁcation and selection of valves, which improves process performance, increased reliability, plant safety and environmental protection.”
To classify the degree of severity, a weighted matrix has been created, where speciﬁc service-related parameters are input and calculated for a ﬁnal score. Some of the determining factors include cavitation, ﬂuid velocity, frequency of operation, process design pressure, process design temperature, toxicity and ﬂammability.
Here is an excerpt from the severe service data input table of the proposed MSS document showing some of the service condition parameters along with their respective scoring:
The new document will classify valves in three categories:
- General Purpose Valve: Valves that are typically produced in large quantities complying with one or more frequently requested industry standards. They are sometimes referred to as commodity valves and are regularly available in distributor stock.
- Fit-for-Purpose Valve: A general purpose valve modified with one or more alterations to part dimensions, materials of construction, factory cleaning, surface preparations, testing or packaging.
- Severe Service Valve: A valve that is specifically engineered for an application.
A NEW MSS COMMITTEE
As you might imagine when creating a new standard, there is much debate on the categories and relative weight that each category carries in reaching the final severity score. As the drafts are tweaked and voted on by the workgroup, the tables are being adjusted. Work on this standard is being performed under the auspices of a new MSS committee: Com. 410, Severe and Special Service Valves. This group also will create standards for valves in other applications.
This new MSS SP will be a useful tool for both the engineer and manufacturer when deciding just what type of valve should be selected for a given set of operating conditions. The document, when used with the American Petroleum Institute (API) Recommended Practice (RP) 615 Valve Selection Guide, should be a great help in valve selection for valve professionals.