Category Archive: Control Valves
Control valves are a single component of a bigger system, but they control fundamental parameters of a process to keep applications performing optimally.
Control valves are used to control fluid passage through regulation of flow, level, pressure, or temperature.
Control valves are commonly used in oil & gas and manufacturing industries.
The efficiency of automatic control valve operations has saved time and reduced wastage across various applications.
Nonetheless, it is essential to keep control valves in check by performing yearly tests and detecting causes of performance issues immediately.
We’ve got a few helpful tips to keep in mind next time you want to better understand why your control valve isn’t functioning at its best.
Check for Leaks
One of the most significant issues regarding control valves are the internal and external leaks.
Internal leaks can be caused by the fact that components are becoming worn out or that the actuator of the valve isn’t correctly set up.
More often than not, there is a specific allowable range of leakage per valve; however, if the valve is leaking more than the ‘maximum allowable leakage’ this indicates a concern that may require some repair or replacement.
External leaks are also prominent in control valves and release damaging fugitive emissions into the air.
External leaks are usually a result of valve packing for improper maintenance, material incompatibilities, and vibration of valve assemblies.
Sticky Valves & Stiction
Minor leaks are often resolved by tightening the packing of the valve.
Unfortunately, this is sometimes done without realizing that excessively tightening the valve packing will cause the valve to stick in place and stop functioning correctly.
Along with over-torquing the valve packing, the valve also become faulty due to issues with the positioner that cause the valve to stick.
Stiction is the final player in a sticky valve situation that happens when the static friction exceeds the moving friction inside the valve and causes valves to stick in a particular position.
It’s just the wrong valve
A recurring problem many operators face with control valves is purchasing the wrong equipment for their application. Sometimes valves are used in service conditions that they aren’t compatible with, causing them to stop working and wear out much faster.
It is also possible that the actuator isn’t the correct size for the valve causing numerous technical issues. Proper calibration of the positioner is also essential to the valve functioning optimally. Understanding what the valve is being used for and its role in the assembly can help avoid purchasing the wrong equipment for the process.
Control valves can be maintained and checked for issues and failures regularly to maximize the valve’s longevity.
Depending on the service, system, and type of valve operation, operators can adequately analyze the kind of problems the valve may be having within the system.
Today’s power generating plants face two major factors: reduced maintenance budgets and competition from renewable energy.
Maintenance budgets are tight, often forcing managers to put off problems that really require serious attention. The increasing popularity of renewable energy sources is diminishing the demand for traditional fossil power generation. As a result, plants are regularly operating below intended capacity or coming off line.
Base-loaded fossil power plants operating at reduced generating capacity or become cycling units places heavy demand on control valves in ways they may not have been designed to operate. When control valves operate below their intended capacity they operate closer to their sealing surfaces than intended.
This reduced flow rate greatly increases the velocity within the valve and increases potential erosion. This is especially true on Feedwater and Boiler Feedwater Pump recirculation control valves. Plant managers can begin minimizing control valve seat erosion and reducing subsequent maintenance costs by reviewing sizing of control valve internal trim.
Trending data is excellent for determining actual operational conditions versus what was recommended in the valve specifications.
Control valves are designed with the following factors in mind:
- Fluid type
- Upstream pressure
- Downstream pressure
- Flow rate
Next, plant engineers, procurement, and maintenance planners should understand where the control valves should operate within the facility:
- Less than 10% open should be avoided since the valve will wear quickly.
- Plan to keep the range of control valve operation between 20% and 80% open.
If trending information on the valve is available, managers can quickly see if they are operating within the proper range. If not, they can provide the control valve manufacturer with new operational conditions and request suggestions on improving low flow performance.
The takeaway is an oversized control valve will ultimately wear more quickly under lower than designed flow conditions. Plant managers can prevent erosion and limit maintenance spending by reviewing and possibly changing the control valve’s sizing. This can be done in one of two ways:
- Replace the internal valve trim with the proper size for new operating conditions, or
- Downsize to a smaller valve.
While a full control valve replacement is more expensive than a trim replacement, both solutions deliver significant long-term maintenance and operating savings.
DFT®’s HI-100® is a competitively priced severe service control valve with a low total cost of ownership. With robust features designed for long service life, the HI-100® will help keep a plant’s maintenance budget in line.
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DFT® proudly offers a regular Control Valve Newsletters to address common industry applications. Applications covered in recent Newsletters include:
- Feedwater & Steam Drum Level Control
- Sky Vent for Combined Cycle Plants
- Turbine Bypass
- Soot Blower Control
- Attemperator Temperature Control
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It’s my weekend off and I get a call from the plant. The level alarm has gone off and our process had to be shut down. It turns out that the level control valve has once again failed in its task. We do our typical “work around” to start up the system and operate the level control in manual until it’s fixed.
It’s the Google age, so I spend Saturday night looking for help on the Internet. Unfortunately, by Sunday morning I am more confused than when I started. I need information that actually helps instead of making things worse—where can I find a solution in order to reclaim my weekends?
Let’s start with the basics – What choices do I have?
Often, the replacement choice is made simply on the basis of the pipe size in the line that is made by removing the old valve. Control valves come in many different configurations, but which one do I need?
- Butterfly Valves use a flat disk that is rotated in the flow of fluid. They are well suited in larger line sizes where a minimal pressure drop (less than 50psi) is desired.
- Ball Valves use a sphere with a hole through the center. The ball is rotated in the flow to adjust the flow of the fluid. They are generally best used in on/off situations.
- Plug Valves use a cylinder to obstruct the fluid flow, and similar to ball valves, they feature one or more holes through the cylinder. Flow is regulated by rotating the cylinder within the valve chamber. Like ball valves, they are good choices for rapid shut off, but they can introduce turbulence into a process system in some conditions.
Rising Stem Types:
- Globe Valves are named for their generally round bodies, and they use a linear rising stem motion. This design is easy to maintain in harsh environments.
- Venturi-ball Valves employ the same linear rising stem motion. Designed for high pressure drops and easy maintenance.
Since the pressure drop through the control valve for this level control application is over 100 psi, I know that I need a rising stem valve.
What Trim Characteristic Should I Use?
- Linear Trim – Changes in position are directly proportionate to changes in flow (a 10% change in valve lift produces a 10% change in flow). Linear trim is used in systems where the pressure drop through the control valve is the major pressure drop in the system
- Equal % Trim – Equal changes in position cause an equal percentage change in the previous flow (a 10% change in valve lift produces a 48% change in flow for each position). Equal % trim is used in systems where the pressure drop through the control valve is generally less than the other pressure drops in the system.
In order to choose the proper trim characteristic, I need to know what I am trying to control.
- Use Linear trim unless the criteria for Equal % trim is met
- Use Equal % if the pressure drop at max flow is 1/5 that of the drop at min flow
- Use Linear trim for slow process changes
- Use Equal % trim for fast process changes
- Use Linear trim for varying set point
- Use Equal % trim for varying load
- Use Equal % trim
Since I am controlling the liquid level in this example with relatively constant pressure drops, I choose Linear trim.
The DFT HI-100™ control valve is a good choice for this application. It is a rising stem control valve with a Linear characteristic. The valve has the Quick Change Trim feature that permits the in-line replacement of the internal trim.
Our process had to be shut down again. It seems that the original valve is no longer up to the task. Should I continue to simply repair the valve or should I look for another solution? Up until now, it was far easier to repair the valve and/or replace it in kind … but my process efficiencies are going down and I need to find a solution. Where do I start?
I pull out the valve data sheet and take a look at the original conditions. Wow! I can see immediately that things have changed –
- The plant has aged nearly 20 years;
- We replaced the pump on that line 5 years ago;
- We have modified the piping in the area of the valve;
- We changed the fluid chemistry;
- Instead of operating continuously, the plant is now cycling
Every time conditions change, we need to review the impact upon the control system.
As a system ages, it does not get any cleaner. Whether it is due to pipe scale or there is other sediment in the system, a valve that worked in a clean system may not work as well in the older system. We have periodically been pulling the valve with cage style trim in order to drill open the passages. We have also noticed that the valve trim is wearing out more quickly. The valve simply requires more maintenance.
Installing a new pump can change how the system operates. A new pump generally means that we have a new pump curve to work with. The new pump curve means that the flows and pressures have changed from the original specification.
We added a new branch line. A closer look reveals (I use my 20D2 rule of thumb) that the new line may have lowered the flow rate through the control valve. Is it possible that the control valve is no longer properly sized? The trim in an oversized control valve erodes more quickly than the trim in a properly sized valve. Alternately, if the valve is undersized, I may be seeing high velocity erosion which also negatively affects trim life.
Although we have always maintained our chemistry within recommended boundaries, we can see that our targets have changed over the years. Newer technology allows us to more closely hit the mark. Is it possible that this has tipped the balance and we are now seeing erosion-corrosion in the valve since the originally specified materials are no longer noble enough?
Cycling vs. Base Loading
When we changed from a base loading operation to a cycling operation, we started to strain our control valves. Now they operate more frequently and often work in the low end of the range. By changing the operation, I now need to have a valve that is suitable for severe service when previously I could use a standard valve.
The DFT HI-100™ control valve can be a good choice when conditions change. It deals well with dirty fluid, can be sized properly for the new conditions, is not subject to erosion corrosion and is designed to perform well in the most severe services.
Proper valve selection is critical for ideal operation of any system. Improper valve selection can cause system underperformance or failure. The greatest problem when selecting valves is incorrect information.
Often, we may only have access to incomplete or dated information regarding valve applications. Do we know enough about the system the valve will be used in or the main purpose of its use? This “bad information” can create a situation where the theoretical situation does not match reality.
Good Information is Key
“Bad information” comes in many forms. Original design specs might be difficult to obtain, and having those specs does not guarantee success. Occasionally, specs aren’t properly updated. Information that was good at first can fail to incorporate changes in process conditions. If the actual pressure, temperature, flow-rates or the media is different than that which was used to select the valve, it may fail or fail to work properly.
When choosing a gate, globe, ball or check valve, we always want the valve to be operating in the full open or closed position. Control valve selection, however, targets 40% to 60% open for normal flows. When most control valves operate below 20% open or over 80% open, they can lose their ability to control properly.
What information is required to properly size a valve? Fluid type, operating pressures and temperatures upstream and downstream of the valve, and the flow rates associated with these conditions are required information to have.
Knowing What is the Right Valve for the Application
There are many different types of valves available. They do not all work the same way or for the same purposes. Each valve type has its advantages and disadvantages.
For proper control valve selection, it is important to understand how the valve is used to control system flow. For example: is the system tuned for a constant set point, or is it set to modulate a variable flow over time? The layout of the piping system and locations of reducers and elbows also impact valve performance.
Often, choosing what seems to be the most efficient valve might not be the best valve for application. Replacing a valve “in kind” with an upgraded version of an older valve might actually do more harm than good.
Poor Performance – Get Good Information
Unfortunately, the blame for poor performance often lands on the installed valve when it is actually bad information that caused its failure. Simply repeating the bad information to select a different valve is doomed to have the same results. When DFT Inc. is asked to troubleshoot an installation, we start by collecting all of the available information and check for “bad information” concerning flow conditions and application mismatches with respect to the installed valves.
Take the time to get good information. Fortunately, the information is more readily available today than it has been in the past through trending information. If you can provide actual system operational information, your chance for success increases significantly.