Subject: Leveraging computational fluid dynamics simulation of capacity, velocity and pressure profiles can eliminate the need for valve testing, enable better insight into applications and provide troubleshooting/optimization opportunities for installed valves.
By Reid Youngdahl and Luke Novak, Emerson Automation Solutions
Reid Youngdahl, Valve Technical Specialist at Emerson Automation Solutions and Luke Novak, Computational Fluid Dynamics Specialist at Emerson Automation Solutions, recently published an article in the March 2020 issue of Hydrocarbon Processing magazine describing the increasing use of computational fluid dynamics (CFD) simulations to predict the performance of control valves in non-standard installations, and to troubleshoot control valve problems in critical applications. A summary of that article appears below.
Real World Problems with Virtual Solutions
Today’s business climate is compressing project schedules even as process designs are becoming increasingly complex. Control valves must perform under very demanding process conditions that may not have been previously encountered.
Because failure is often very costly, many end users require the control valve vendor to flow test critical valves under the anticipated conditions to ensure that they will work as designed. Unfortunately this effort takes time and can be prohibitively expensive. For larger valves or very extreme process conditions, these tests may be practically impossible. However, the authors explain another option:
The alternate solution is simulation. With computational fluid dynamics (CFD), a simulation engineer can develop predictive, physics-based computational models to cover a wide range of tests and situations including valve flow coefficients, multi-phase applications, turbine bypass temperature sensor optimization, installed valve troubleshooting, among others. With CFD, a simulation engineer can input the design of the valve in question, simulate the installed conditions, identify the problem and develop a solution.
By modeling existing process conditions, CFD helps designers formulate solutions to control valve issues such as buffeting, vibration, erosion, valve instability and others.
CFD at Work
Most valve vendors use CFD simulation (Figure 1) as part of the design process, so the advanced equipment physical models are already available and have been paired with a variety of dynamic flow models to predict a valve’s performance. Some vendors are now offering those advanced simulation capabilities to their customers so they can virtually test non-standard installations, and replicate and troubleshoot problems in the field.
Figure 1: This diagram depicts a fluid flow distribution through a valve as predicted with CFD.
The authors further describe the advanced offerings of CFD:
Some simulation capabilities include:
The authors went on to describe five examples where CFD has recently been used to save end users significant time and significant expense, and/or solve ongoing installation problems. A summary of those cases is provided below.
CFD Replaces Flow Testing
A major OEM of power plant turbines required flow capacity validation on a 32-inch butterfly control valve (Figure 2) installed in a critical bypass application.
Figure 2: CFD simulations proved that this Fisher Model 8532 butterfly control valve would pass sufficient flow for the OEM’s turbine application.
Traditionally a flow test would be required, but the increased costs and negative schedule impact were unacceptable. Instead, Fisher valve engineers created a CFD model of the butterfly valve to simulate the Cv. The simulation provided a ±7% confidence band for all flow coefficients. By using CFD as an alternative to physical flow testing, the turbine OEM saved over $100,000.
Accommodating Actual Installations
Thirty years ago, a refinery installed a severe service control valve (Figure 3) in the non-preferred flow-up direction. A capacity expansion project required a larger valve, but plant personnel wanted to use the identical valve type due to its 30+ years of proven service. Changing the piping to the preferred flow-down orientation would be costly and take an excessive amount of time, so the refinery wanted to keep the orientation flow-up. However this brought the valve sizing into question since the sizing parameters were only flow tested in the flow-down orientation.
Figure 3: A refinery was installing this Fisher Type 461 severe service control valve in the non-preferred direction. How can it be correctly sized?
The application was too critical to size the valve by extrapolating valve sizing parameters, so a CFD simulation was performed to predict valve flow coefficients and pressure drop ratio factors. The valve was correctly sized and has been operating as planned since installation.
Temperature Sensor Location Optimization
Turbine bypass is a very critical control valve application in a power plant. Properly selected turbine bypass valves (Figure 4) keep a turbine safe and maintain overall heat rate by de-superheating the bypass steam with water injection.
Figure 4: View of a steam recycle valve.
CFD was used to predict the ideal location to install the downstream temperature transmitters. The revised locations allowed the plant to reduce downstream straight-length piping requirements, cutting installation cost and time.
Sizing/Troubleshooting Valve Actuators in Critical Service
A high-pressure injection pump recycle valve on an offshore platform was experiencing instability issues when trying to control a 3700 PSI pressure drop. The solution to address this critical, severe service control valve was using CFD to ensure the control valve and the downstream pressure relief valve were sized correctly. CFD was also used to perform a root cause analysis that determined that the actuator was undersized. An actuator replacement solved the problem.
Multiphase Sizing Application
An OEM of water treatment skids required CFD analysis of its severe service, multiple component, fluid control valve to ensure it was sized correctly. The OEM did not want an oversized valve as this would require a larger downstream pressure relief valve. OEM process engineers provided full fluid composition data, enabling the valve simulation engineers to generate a CFD report and correctly size the control valve, minimizing the overall cost of the skid.
CFD Conclusions
The authors summarized their article by saying:
When a valve vendor designs a valve using CFD and tests the designs in a flow lab, it produces a software model that can be used to predict or diagnose problems with installed valves. CFD simulations are now being used in not only new product development, but are also being provided by valve vendors as a service to end users, saving time and money when diagnosing problems with installed valves.