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.
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.
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.
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.