A common problem in process industries is check valve failure that can and does lead to additional equipment being damaged due to internal check valve components flowing downstream into process vessels and valves. In addition to this, in extreme cases it can cause an operation to rewrite operating procedures to increase flow and address situations when the process needs to be stopped.
CGIS’ commitment to our clients and partners is understanding the application, well before a dollar is spent to ensure the client always gets the right valve. We are leading the charge to define Severe Service and believe this is critical because we can and should expect more from industrial valves today.
The purpose of the following example and review is to identify the many factors that need to be considered in selecting check valve design and sizing check valves for an application.
It was discovered at a large gas processing plant in Northern British Columbia, where following a turnaround the flowrate from the facility to the sales gas pipeline was at a significantly reduced volume. The operations sprang into action to open bypass lines that would be closed in normal operating conditions to increase and provide enough flow into sales gas pipeline. The facility could not afford to shutdown at that point and the decision was made to continue operating with open bypass lines, which led to new temporary operating procedures being implemented to have a plan in the event of an unplanned shutdown of the facility.
A review was initiated to determine the cause of the reduced flowrate, which eventually pointed to a check valve upstream of a modulating pressure control valve. This specific check valve was being replaced on a routine basis during each turnaround that occurred on an average of every 3 years. Various types of check valves had been in service through the years to improve service life and performance.
The initial hypothesis was on the right path and is the correct thinking for selecting a check valve, which was the installed check valve provided too low of a Cv (flow coefficient) and the pressure drop across this valve was too high thus affecting the volume of flow.
CGIS was asked to provide a sizing review of the application and provide a recommendation; we were provided with flow data giving us minimum, normal and maximum operating conditions for pressure and flow rate along with the physical data of the gas. In addition, the client stated target Cv of 3,000 and low pressure drop of <69 kPa (10 psid).
The process information provided was as follows;
Based on the information provided and sizing results, it was determined that a 20” Class 600 valve with a venturi inlet would be optimum for this particular application. In this case, we recommended a Hy-Grade Series ‘T’ API 594 wafer swing check valve. The results of sizing calculations were:
700 MMscfd Flowrate
450 MMscfd Flowrate
The client then asked us to prove the results of our sizing calculations to verify these would meet the needs of the process engineering and operations teams and ultimately, performance of the gas processing facility. CGIS coordinated third party flow testing of the valve prior to delivering the valve to the clients’ facility.
The venturi inlet increases the velocity through the valve to raise and hold the disc in the fully open position to meet the required pressure drop and Cv (flow coefficient) targets. Through the third-party flow testing results, the criteria set forth by the client were met with a Cv of greater than 3,000 and a pressure drop of less than 69 kPa (10 psid) pressure drop.
After a thorough review of the process conditions and requirements, on the day of the installation we discovered the truth around the reduced flowrate. When the plant starts up, they start moving gas at 5,515 kPa (800 psig) and it hits this valve to open. The check valve disc had sheared away from the hinge and travelled into the downstream pressure control valve, thus blocking flow to the sales gas pipeline. The failed check valve was now a spool piece in the piping and its disc was affecting flow through the pressure control valve.
As you may, the average life span of previous installed check valves averaged three years. The 20” Hy-Grade Series ‘T’ check valve was installed in 2011 and remains in service eight years later operating in the process efficiently and without mechanical failure.
As mentioned, the purpose of the above example is to bring awareness to the many factors that need to be considered in selecting and sizing check valves for an application including the valve design.
There are sub-categories to the general term Check Valve or Non-Return Valve. The sub-category designs of check valves will suit specific applications and provide performance benefits to liquid and/or gas media along with flow or application conditions, such as laminar or pulse flow.
There are six specific types of check valves;
In the application example, internal components of the check valve must contend with normal process flow requirements and extreme start up conditions. Certain designs are more susceptible to violent conditions such as quick opening or quick closing; the solution to the above application was in a Single Flapper Wafer Swing Check Valve utilizing a one-piece disc, shaft and hinge as shown below.
In comparison, a Bonneted Long Pattern Swing Check Valve or Dual Disc Check Valve have multiple connections and components that need to contend with the extreme opening and closing cycles
The long customary practice of check valve installation is based on the pipe size and pressure class and choosing a valve of the same size and pressure class, which is typically the reason for premature check valve failures.
Check valves share the same sizing requirements of modulating control valves, where solving for Cv (flow coefficient) is needed to ensure the check valve operates efficiently. The flow through the valve moves or lifts the modifier to its fully open position, whether this is a ball, disc, plug or other design type. When in the fully open position, the valve will have the least affect on pressure drop or decreasing through flow. Check valves function by the energy of flow to open or close.
In the case of liquid flow applications, a Bonneted Long Pattern Swing Check Valve requires 11-12 feet per second of liquid velocity to reach the fully open position. In most liquid flow processes, the average flow velocity is 7-9 feet per second. In an engineered Single Flapper Wafer Check Valve with a venturi inlet, we can size the internal components to provide enough flow velocity to raise the valve into its fully open position allowing us to select a size and pressure class to match the pipe size and class.
When a check valve does not fully open, the modifier (disc as above figure shows) will be partially open and be affected by flow. The affect of the flow creates damaging conditions through chatter, slamming and vibration along with turbulent flow conditions that can twist or bend components. These conditions have an impact on the service life of the valve through the sealing components (disc and seat) damage along with assemblies working themselves apart. That is, the bolts and/or nuts that connect the various components.
More importantly, the affect on process balance brings down the overall efficiency and requires more energy to operate.
In summary, it is important to properly size based on application review rather than choosing a check valve that meets the line size and pressure class of the pipe. Reliability will be greatly enhanced when this approach is utilized and lowest cost of ownership will be achieved through lower repair/replacement; most importantly though, are the energy gains in having an efficient and reliable check valve product. As in the case of my application example, we have almost tripled the life span over previous designs.
Kevin Niebergall for LNG Industry October 2019
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