Ten things to remember when buying control valves

Nov. 6, 2006
Most valves serve harsh lives, either from the fluids handled or the demands of service. Follow these 10 tenets when purchasing new or replacement control units.

A Readers Digest story once described a fellow hiking through Japan who came upon a man in a field working on an irrigation sluice diverter plate. The sluices had been blocked, the stem support frame removed and the handwheel, support bushings and even the slide rails had been removed and cleaned. The support steel had been repainted. Reassembly was in progress, all done with applications of lubricants to parts kept clean throughout the work.

The hiker complimented the worker on his attention to detail, and asked how long the control plate had been in service. The response was a questioned look. The worker explained that he had been doing this annual ritual on the valves in his fields for 10 years, following his father, who had done it for the previous 35 or 36 years, and he had learned what to do from his father who had done it for an unknown number of years before that.

A little maintenance can make for significant longevity. But not every flow control application provides the conditions available to our Japanese friend. Most valves serve somewhat harsher lives, either from the fluids handled or the demands of service. That’s where I hope this article will help you.

Having experience in the fluid handling game in a variety of industries and as many service fluids, from benign to aggressive, I offer some considerations that can assist you in selecting new or replacement control units.

Selection isn’t simple

The Japanese example represents a simple system — water in an open channel, limited flow, upstream generally limited by other diversion, low pressure, frequently under observation, and a few other basic criteria. Industrial valve applications are rarely that simple.

While basic applications can be handled easily, sometimes we’re called on to select components for systems few have considered or know about. For example, the Italian engineer who devised the water distribution system for the garden fountains of Tivoli also faced unique challenges. His jump to the next level of technology introduced control by orifices and constant levels.

We can see the game getting more interesting. New applications bring new pressures, temperatures, flow rates, control accuracies, solutions and mixture conditions that might cause us to paraphrase Thomas Paine: “These are the [i]things[i] that try men’s souls.” So, let’s start making the job a bit easier.

Seek reliability

The valve has been a reliable flow-management device for several hundreds of years. The Italians are still uncovering cast plug-type valves that were used on Caesar’s barges. Roman aqueducts controlled channel flow. Dams and water wheels have been part of our own history. Over the ages, we’ve developed a variety of valves to serve many purposes.

The first were basically to stop or start flow.

There are reasons for choosing the valves we do. When we select a valve, we rely on guidelines we hope will be justified by good operation and long life. A key element in any of these choices is reliability.

Consider the purpose

A gate valve is a start-stop unit. The fluid must be clean enough to prevent build up in the cavity where the gate fits. If the gate won’t seat, the valve won’t stop the flow. This concern led to double-block-and-bleed valve configurations in critical areas. Plug valves, while great for limiting clean fluid flow, have been destroyed by fluids that contain sand or other grit that erodes seat rings.

The point is correctness of application. Vendors spend time and effort asking detailed questions to assist selection decisions, but operations people often recognize that a given piece of data, by omission or commission, can make the difference. Keep the ultimate user in the loop to get better application selections.

Specify suitable materials

Part of a good selection is materials, both of manufacture and application. While valves of durable materials are available, the method of manufacture also might be of concern. An alloy casting process that produces fine-grain valve surfaces should prove beneficial.

Corrosion resistance seems to correlate with smaller grains on the exposed interior surfaces. Plastic coatings have negated some of this issue, but surface condition still must be considered.

Using test coupons in a given stream is a good way to confirm metallurgical suitability. Some argue that the method by which the coupons are connected can influence the test results. For a particular chlorinated organic application, engineering references suggested several suitable materials of construction. The manufacturing process, however, left a corrosion residue on the coupons. Testing revealed that what seemed to be a relatively pure stream contained enough of the corrosion promoter to restrict use of the suggested materials. The final choice was polymer-lined valves.

Location, location

Valve accessibility is an important variable. Placing one where it can’t help but accumulate even a small amount of solids makes failure probable. Installing a valve in a downward vertical run upstream of an elbow is far superior to placing it in a horizontal run just after an elbow.

Placing a valve far from structural support exposes the line to strains from operation shock. One plant with mostly polymer piping has a corporate guideline that says valves should be supported rigidly. As a result of following this rule, the lines between them need little concern except for longer runs.

Smooth operation

The manner of valve operation also is relevant. Your choices are more a function of the actual piping layout and arrangement than the P&ID. This selection is often a field choice during plant construction. Fast-track installation directly from P&IDs means the owner’s jobsite representative must be cognizant of the needs of operation to make proper choices. Placing an automatic operator close to walls or where movement can’t be observed can lead to production delays and maintenance headaches.

Valves with rising stems can bring hand-pinch problems, quarter-turn valves can be knuckle busters. While these issues are rare in well-developed plants, our old friend Murphy tells us that when they arise, it will be in the worst place at the worst time. We’ve all seen drain valves for dangerous liquids installed so the discharge points directly into a wall or equipment base where splash-back is a serious safety concern.

Pick appropriate seals

Stem seals or packing is another source of concern because a seal that binds or dissolves in the service medium is an obvious problem. Also, anything that can damage packing or score a shaft is important. This brings us back to the application, and the circle can start again, often with additional possible side branches for more selection questions.

Consider time and frequency

Frequency of operation can drive a sane choice crazy. A while ago, I had to consider how much movement there was on a control valve. The first answer was “None. It’s in control, so net movement is zero.” But after research and discussions with control valve vendors, it became more reasonable to suggest a 10% movement on a 30-second cycle. This led to a better understanding of how much motive effort was needed to maintain control and an appropriate design for the pneumatic part of the control system.

The frequency of manual operation also is a factor. If once-a-year operation is typical, and the location isn’t ideal, manual operation might be acceptable, but for weekly or monthly operation, you might need to provide another appropriate method. Remote manual operators have been worthwhile investments for some of these situations. They can prevent complicated drop legs and high pressure drops that continuously absorb energy.

Avoid entrapment

If you’re designing a slurry system, don’t place valves near dead-end turns and in long vertical lifts. Maintaining fluid velocity is key, but follow-up flushes can prevent some problems.

Clear liquids aren’t immune. Consider a vessel feed system where several liquids enter through the same nozzle. If the fluids react or the solutes undergo a phase change, reliability can be helped with a block valve at the nozzle or a feed system flush using the bulk fluid in the vessel.

Phase changes can subject valves to significantly different conditions that might not be obvious. Boiler blowdown, for example, often discharges directly to low-pressure lines. The combination of scale buildup, suspended solids and sonic velocity from flashing discharge can form a grinding medium. A bit of line length downstream of the valve can help retain liquid conditions and reduce velocity through the valve. This downstream flash chamber is a lot easier to replace than a blowdown valve.

Consult operators and technicians

Whether you’re upgrading an existing system or designing a new one, get to know the operators. Talk to them about the suitability of the valves currently in use to help determine which types are reliable and which aren’t. Remember the oft-neglected truism that the most expensive valve is the one that burns up more time each day than it’s worth.

Having said all this, it’s still difficult to provide a unified checklist for valve selection because new applications often bring new problems. Vendors are still a great reference source, but operations and plant maintenance personnel are closer to your situation and should be consulted. An unscheduled shutdown can easily overwhelm any savings derived from an inadequate valve. Reliability is the aim, because plants make money only when they run.

J. Edward Hardin owns Hardin Consultants in Charlotte, N.C. contact him at [email protected] and (704) 841 7384.

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