Valves & Automation
Are your control valves oversized?
Control Valve Size Matters
“A survey of more than 500 individuals involved in process engineering, procurement, operations and maintenance at over 200 plants worldwide identified “oversizing” as the number one control valve problem.” Michael McCarty, Emerson Process Management
A valve that is too small is immediately obvious.
However, an oversized valve may look like it is operating normally and the user is unaware that it is costing more to operate & maintain. So what is the impact of an oversized valve (and actuator)?
- Bigger valves and actuators cost more.
- Larger equipment in the process loop drives up cost. i.e. safety release valves, isolation valves, bypass valves and drain valves
- Energy costs increase as a result of exaggerated pressures and/or flows that were used to size the pump or compressor.
- Additional cost results for pipe and installation.
- It is difficult to impossible to control the process. Small valve position changes result in large changes in the process variable.
- Any stickiness (friction or dead band) in the valve exaggerates the sensitivity (process gain) and prevents optimal loop tuning and performance.
- The higher flow expectation for the pumps and compressors, means more energy or horsepower is absorbed by the valve and piping system.
- An increase in the maintenance cycle can result from an oversized valve operates in a lower opening intensifying the wear and erosion to the valve seat or seal, valve trim and even the valve body.
So how does a valve get oversized?
Besides the obvious errors in process flow calculations or valve sizing calculations, consider this scenario:
- The process engineer figures out the maximum flow required at the pressure needed and adds a 10% safety factor.
- The process engineer gives the maximum flow requirement to the mechanical engineer who is specifying the pump requirements.
- Just to be sure, the mechanical engineer uses a 10% safety factor when sizing the pump pressure and flow requirements.
- He now forwards the updated flow requirements to the control engineer
- The control engineer now adds his 10% safety factor just to be sure the control valve is large enough.
So how can we solve this problem?
Simply put……the control valve needs to operate 60% to 80% open at the maximum required flow rate and not less than 20% open at minimum flow. You should use as much of the valve’s control range as possible while maintaining a reasonable, but not excessive, safety factor.
Do you measure or not?
Automating an Existing Value
Does This Sound Familiar?
“We reserve the right to make changes in design, materials, dimensions or specifications without notice and without obligation to make changes or modifications on previous or new products sold.”
– Brand “X” Valve Supplier
As companies automate more, sometimes is makes sense to automate older valves.
Sounds simple. Sometimes it is.
But often things aren’t as simple as they sound.
The first thing to consider:
Is the valve suitable for automation?
- Does the shaft have enough space to include a coupler?
- Is there a level surface to mount the automation hardware?
- Is the valve obsolete?
- Would it cost less to replace the valve and keep the old valve as a spare?
Once you have decided it is possible and cost effective to automate the valve in the field, you may decide to consider one of two options:
You take the simple (and cheapest) approach.
- You take a picture of the valve in service with your smart phone.
- Take a few measurements with your tape measure.
- Get the model code of the valve or find the valve manual in an old bookshelf.
- Call your favorite valve vendor.
- Talk to the vendor answer a few additional questions enter a PO and then wait.
The completed assembly finally arrives.
During an unplanned shut-down in the middle of the night, you decide to automate the valve.
You immediately discover the mounting hardware doesn’t fit.
You determine the bolt-hole centers on the actual valve are different from the valve drawing you had which had the above disclaimer statement.
You send the assembly back to the vendor, have the vendor rebuild the mounting bracket or drill new holes if possible and again wait for an unplanned shut-down.
“If” you get lucky, everything comes back and fits the second time. (or maybe not)
This field technician is measuring eight manual ball valves that need limit switches. All eight valves are the same (manufacturer, model #, size and pressure class) but there are three unique sets of measurements. Three different automation designs were required.
You decide to do it right, especially since you are the new kid on the block.
You call a vendor who is recommended by a trusted friend.
A qualified machinist arrives at your site with calibrated calipers and micrometers. Hopefully the machinist is the same person designing the mounting hardware.
He takes a number of critical measurements. The measurements often include the flat areas of a double-D or square shaft or the diameter of a round shaft with key ways.
The last thing you want is a situation where the actuator shaft turns first, then the coupler, and finally the valve shaft.
As an added bonus the machinist is familiar with valve designs and proper valve function. He is trained to locate any horizontal and vertical clearance issues.
Often pipes or structural materials can be moved but valve orientation must be changed from vertical to horizontal or vice-versa.
The new assembly arrives at the plant.
During the shutdown, with the machinist on site, the new assembly is connected and tested.
It works great.
It is a very good day.
Commissioning and start-up assistance is critical
Make sure the measurements are correct.
Also validate fit and function to provide a long-life.
Use a qualified technician for a “test drive” and give site maintenance and operations a chance to experience hands-on training.