How I Saved the World and Then Woke Up: The importance of safety relief valves
September 01, 2007
Appeared in International District Energy Association
I truly hope your summer has been fantastic. Mine has. Along with getting sunburned and working hard at my new job, I have seen a bunch of movies: “Spiderman 3,” “Fantastic Four: Rise of the Silver Surfer,” “Pirates of the Caribbean: At World’s End,” “Harry Potter and the Order of the Phoenix” and more. I just love movies, but I’ve seen so many that I’ve started to have weird dreams – really weird ones that combine my own version of the movies with features from work. Take this dream for example:
I am miniaturized and in a mini-sub like the one in “Fantastic Voyage” (but without Raquel Welch). The sub is called the Black Pearl, and I am getting pushed along by system flow alone (got to conserve energy with the price of diesel these days!) in the world’s largest district cooling water pipe system. (I must be in Dubai!) My job is to troubleshoot any problems and look for maintenance issues. It is a really exciting pipe layout with many bends, like the water parks in Wisconsin Dells, but on steroids (once again think Dubai). Yeehaa! My external sensors indicate that the water conditions are 42 degrees F and 100 psig. Cool – everything is normal, and we are flying like the Silver Surfer.
All of a sudden, the water flow terminates, and I coast to a stop. Hmmm.... The temperature is still steady at 42 F, but the pressure has dropped to about 50 psig. Either I ran out of quarters or the pumping system has been shut down by the dreaded Sandman! As I get my bearings, I discover that I am in a plate heat exchanger (PHE), and the isolation valves are closed so I am not going anywhere. I am trapped!
After about an hour of cruising around the herringbones of the PHE looking for an exit, I notice my external temperature sensors are slowly rising from the standard 42 F to past 45 F. Curious, but I am nice and comfy in the Pearl, so I do not give it another thought. After a while, however, I am bored silly of flying like I am in a Quidditch match, so I do the natural thing when the going gets tough: I take a nap to recharge. (Hey, Quidditch is hard work!)
Several hours later, I am jolted awake by the pinging and popping of the Pearl. I check the instruments, and the water temperature is up to 60 F. Great Caesar’s ghost! What’s happening out there? The pressure gauge is reading over 400 psig! How did that happen? My sub is only designed for 500 psig, so I have to work fast.
Bringing up the system schematics quickly on the graphic display, I see that I am close to where a safety relief valve (SRV) between the PHE and the isolation valves should be. Why hasn’t it popped yet? Navigating to the SRV location, I see that it has been removed and replaced with a cap. Foiled again by my arch enemies! But in an inspiration, I take out my magic wand and mutter “alhamora” and the cap opens! I am purged out of the system like a slug coughed up by Ronald Weasley.
Floundering on the wet energy transfer station room floor, I utter another hex, “Engorgio,” and once again I am full-sized. I pocket my trusty wand and review what just transpired. After a moment, it comes to me… Of course! This is exactly what they teach at the ITT Fluid Handling Little Red Schoolhouse – even a small increase in temperature will increase the system pressure dramatically. Those SRVs are really worth their weight in gold!
I reinstall the SRV and save the world…err, at least the chilled-water system. Just then Sue Storm of the Fantastic Four materializes out of thin air and is going to give me a congratulatory embrace, but then I wake up. (Dang!) I guess I have either been out in the sun too long, or I really have seen too many movies. However, within all these extremely corny babblings is real live story.
Unlike me, SRVs are really the ultimate pessimist. They just sit around and wait for things to fail, but before the system reaches that critical point they spring into action to save the day. Not only are SRVs critical to protecting piping systems and equipment, but more important, they protect the safety and well-being of operating personnel.
Not only are SRVs critical to protecting piping systems and equipment, but more importantly, they protect the safety and wellbeing of operating personnel.
It is true that a minor increase in temperature may create more extreme increases in pressure. That is exactly why the SRV is located between the isolation valve and the PHE connection. Even a small movement of the safety valve will relieve a great deal of pressure. One article I came across lately used an example of where trapped water between block isolation valves in a 2-inch Schedule 40 carbon-steel pipe will reach an ultimate pressure of 1,200 psig with only a 30 F temperature rise or 40 psi/degrees F. That is, of course, if no SRV is present.
The terms “safety valve” and “relief valve” are frequently used interchangeably. Both are designed to prevent excessive internal fluid pressure buildups; both are spring-actuated and very similar in appearance. Some authorities restrict the term “safety” valve to boilers or fired vessels installations, with all others being classified as relief valves serving liquids. In addition, here in the U.S., “safety relief valve” has a dual meaning as it performs as a safety valve (quick-opening or with a popping action) when used in compressed gas systems, but also performs as a relief valve by gradually opening in proportion to the overpressure in liquid systems. In other countries, the definitions differ, depending on the codes enforced.
Not everyone has a trusty wand from Ollivander’s in their toolbox; therefore, SRVs have to be maintained periodically so their seats do not stick or become debris-clogged. Many institutions bench test the valves on a three-year rotation and reinstall them if they pass. Furthermore, they are visually inspected on an annual basis for proper orientation, possible corrosion, etc. Manufacturers recommend increasing system pressure to within 70 percent to 75 percent of setpoint and operating the hand lever every six to 12 months to check the SRVs’ condition. Valves smaller than 3/4-inch are not recommended since they can collect debris due to normal testing and use, which can clog the passages.
SRVs have to be maintained periodically so their seats do not stick or become debris-clogged.
So if I promise to get out of the sun and write more serious columns, will you make sure that you locate and maintain SRVs properly? With such an eventful summer, who knows what the winter will bring! I hear the sequel to “National Treasure” is coming out along with a movie titled (believe it or not) “Run, Fat Boy, Run.” That should stimulate some weird dreams – or at least another goofy column.