Valve specification and selection is critical to the success of a newly designed system. A considerable amount of time is invested and dedicated to specifying the correct valves for the defined operating conditions and piping arrangement. Equally as important to consider are undefined conditions such as capacity changes, product changes, and operational variations. These undefined variations often cause problems in the system that require detailed analysis and may require a costly production shutdown to resolve the issue.
By Lea Clauson, Technical Marketing Engineer – DeZURIK, Inc.
The undefined operating variations may relate to startup and shutdown processes, cleaning processes, and emergency and upset conditions. These undefined operating conditions are typically more severe than the actual defined operating conditions. Additionally, there are expectation variations between isolation and control valves that should be considered as well as process growth capabilities. As a result, the following undefined and overlooked considerations should be assessed during valve specification to optimize valve operation and promote system performance.
Startup & Shutdown Conditions
Many valve sizing programs can predict damaging situations such as cavitation, flashing, and noise under various flow conditions. A misleading assumption is that startup and shutdown conditions are rare events and will not harm the valve. If startup and shutdown flow conditions are excluded from the valve sizing analysis, then the selected valve may fall out of the capability range during startup and shutdown and perform poorly.
It is common for startup and shutdown concerns to be addressed with a control scheme during initial system commissioning. The specification process usually does not account for short-term, potentially damaging, service conditions such as pump control. Pump control is not typically addressed in the control scheme. For example, a low-concentrate slurry may have little effect on control valve surfaces under normal operating conditions. But during startup, the control valve may be intentionally left slightly open to prevent pipe damage from filling the system too quickly. During this startup, the increased velocity across the seat of the valve causes the slurry to erode the internal surfaces of the valve, thereby prematurely degrading the valve and impacting its performance and longevity.
Cleaning
If overlooked, cleaning processes can have a dramatic impact on valve performance. This is especially true with valves using elastomeric components for sealing. As cleaning processes typically incorporate steam or chemicals to clean the lines, care must be taken in selecting materials that are compatible with the cleaning media and process conditions. It is best practice to ask about the cleaning process during valve specification to avoid compromising valve integrity.
Emergency and Upset Conditions
Loss of power and emergency shutdown procedures also impact valve operation. In the case of isolation valves, manual overrides may be required for critical applications when solenoid valves are inoperable due to power failure. Control valves are often equipped with spring-diaphragm-style actuators that have a mechanical fail position. However, control valves can also utilize double-acting cylinders or electric motor actuators. These actuators will fail in “last position” upon air or power failure. Because terminology and understanding are not universal, it is important to clearly define what should happen during air or power failure. For example, during power failure the valve positioner will lose signal, thus driving the output to fully open or close the valve depending on designation. Although the air compressor will also lose power, residual supply pressure in the system can continue to cycle the valve to an undesirable position.
Upset conditions including pressure, temperature, and media abnormalities that may occur during an upset event need to be considered and reviewed. Surge protection may be required for pressure upsets. Elastomer selection may be affected by temperature spikes. Slurries may settle and plug piping, potentially causing a variety of issues from spills to dangerous exposure.
Isolation Valves
Valve manufacturers reference applicable standards in their documentation but do not typically publish the actual test criteria from the standard. Instead, they state whether the valve meets or exceeds the standard. These standards are published by organizations such as the Manufacturers Standardization Society of the Valve and Fittings Industry (MSS). They define the test procedures and acceptable leakage rates for various valve types.
It is the engineer’s responsibility to specify the correct valve type for the defined criteria on the application datasheet, but they should also have a clear understanding of the isolation requirement. If a testing standard meets the application criteria, then it may be as easy as selecting a valve that meets that standard. However, when considering a knife gate valve for isolation, the valve selection decision may not be as clear.
MSS-SP81 standard states that a unidirectional metal seated knife gate valve is allowed to leak 40 milliliter/minute (ml/min) per inch of valve diameter with 40 pounds per square inch (psi) line pressure against the gate. With an elastomer seated, unidirectional valve, the leakage rate must be specified or may refer to MSS-SP61, which has a lower leakage rate. If the system has only minimal line pressure available to assist in seating the valve (less than 40 psi), this should be communicated to the valve manufacturer so that testing to actual service conditions can be conducted or a bidirectional elastomer seated valve may be considered. Additionally, manufacturers will often state shut-off capability in language that exceeds the minimum standard, such as zero leakage, but the test results should be evaluated based on their testing criteria.
Specialized Isolation
A valve that is used for isolating a bulk feeder from a hopper under normally flowing conditions will not be able to shut off under a static condition or through a standing column of media. Under normal flowing conditions, a
standard knife gate would slice through the flowing column and isolate. However, if the bulk feeder stalls and the column fills, the standard knife gate will not be able to successfully cut through the column of material to adequately isolate the feeder from the hopper for maintenance. Other possible valve solutions would be an O-port knife gate or a specialized, tapered body knife gate with space fabricated into the valve body to displace the column of material as shown in Figure 1.
Double block and bleed or double isolation and bleed valves are often specified for increased safety in many industries. These valves provide a detection port to verify acceptable shutoff from the isolation valves. It is important to consult the valve manufacturer who is familiar with this style of valve and application since terminology, understanding and expectations can vary greatly.
Control Valves
Control valves, with a few exceptions, should not be expected to perform double duty as a control valve and an isolation valve. Control valves are available with various seat options for different media and application requirements. For example, soft seats such as polytetrafluorethylene (PTFE) for food service, metal seats for scraping the ball or plug in scaling services, and other special seats for critical or unique capabilities such as fire-safe requirements are available. In some applications such as slurry control, the flow control component may never contact the seat and is referred to as a “clearance seat”. These variations in seat requirements may make the control valve unsuitable for isolation.
Future Flow Requirements
Occasionally, a control valve is sized for current conditions with the idea that it will also work for future planned expansions and alternate product runs. The assumption that the selected valve will work for future expansion may produce unsatisfactory results as actual service conditions may be outside the ideal control range. A better solution is to specify a control valve with multiple internal, replaceable control components to accommodate the current application as well as future expansion.
Case Study
Recently, a process facility selected a rotary control valve that could be equipped with four interchangeable flow capacity range seat designs as shown in Figure 2.
This allowed the facility to install a 2-inch rotary control valve body for acid flow control as shown in Figure 3. When the facility was ready to increase flow capacity, it simply changed the trim size of the valve seat. This simple component change eliminated an additional valve purchase, labor costs and expensive piping changes.
Conclusion
When specifying and selecting valves for a new system, it is essential that as much time and attention be given to the undefined or overlooked operating conditions as is given to the defined operating conditions. The undefined operating conditions such as startup and shutdown processes, cleaning process, and emergency and upset conditions may be more detrimental to the valve than the defined operating conditions. If overlooked these conditions may require a costly shutdown to execute a detailed analysis to resolve the problem. It is also important to understand valve requirements related to isolation or control and if the control valve is expected to accommodate process growth changes. Requesting this additional information during valve specification assures the most suitable valves are selected for handling both defined and undefined operating conditions. This proactive approach ensures optimal valve selection for the newly designed system.
ABOUT THE AUTHOR:
Lea Clauson is currently Technical Marketing Engineer for DeZURIK, Inc. (DeZURIK.com) and has worked at DeZURIK for over a decade in various application engineering and materials engineering roles. She earned her degree in Chemical Engineering from the University of Minnesota in 1995. She can be reached at lea.clauson@dezurik.com.
