Steam is used across a wide variety of plant installations, whether for process heating, for cleansing and sterilising medical equipment, for direct reaction in chemical manufacture or for power generation. Dry steam transfers heat readily and needs no circulation pumping, so a crucial part of any steam-producing network is the steam trap, which collects and expels any condensate and non-condensable gases (NCGs) such as air.
But these traps can get blocked, or fail open, wasting steam. With energy prices climbing steeply, the cost is considerable: “You’re talking now typically £30 to generate a tonne of steam, so one trap passing live steam becomes an expensive commodity,” says Andy Cull, national sales manager at Gestra UK. Cull gives the example of a nuclear power station where a single trap was passing steam at a cost of £48,000 a year.
And, of course, a steam leak can be dangerous. But such leaks are not always noticed at first, as they may not affect the performance of the system. A blocked system may be noticed more readily than a leak: “It’s about the production performance of the plant,” says Michael Povey, general manager at TLV Euro Engineering. “If a trap is blocked, it’s not letting any condensate out, so you’re effectively stopping heating happening in that area.” Trapped condensate can also lead to damage from water hammer.
Trap selection is vital, says Povey: “You need the right trap for the right application. A lot of the incorrect traps will work, but they won’t work as effectively or efficiently, and it will affect your process. From our point of view, what customers want is this much heat, at this kind of temperature.
“To get the best-performing trap population, you need to get the right traps and have them installed correctly,” says Povey. He gives three principles for successful installation:
How do you decide how frequently to inspect a trap? “It’s really the criticality of failure,” says Povey. “Most sites would be annual; some do it every six months. When we do a first survey, we typically see a failure rate of 25-30%, and the customer may be thinking their traps are not that bad. But with a good trap maintenance system, after five years we would expect to see that failure rate at less than 5%. It’s more a programme, rather than just a survey, that you need to be implementing.”
SURVEYS
Steam traps can be fitted with devices to allow monitoring in place, from sight glasses to electrodes which show if they are passing live steam or condensate. Wireless units such as the Spirax Sarco STAPS ISA100 unit (see box) can provide a real-time monitoring network.
Michael Povey insists that professional survey can be highly effective and surprisingly cheap: “Look at a 50-trap site with a 20% failure rate: that’s ten traps – you’re losing probably £10,000 a year,” while the cost of such a survey might be between £400 and £800. “We get through nearly 100 traps in a day.”
Andy Cull of Gestra has been running steam trap surveys for many years: “Most customers’ traps are monitored using a combination of an ultrasonic tester and a thermal camera. We put in a pressure (which gives a temperature reading) and when you put the test probe on the discharge point of the steam trap, it takes a ‘voiceprint’ of what’s going through the trap,” says Cull. “If it’s blowing live steam, then it’ll be a very high, straight-line value.
“If you have a trap which you think is passing, or may be flooded or blocked, you can then use a thermal camera to see the temperature differential. Knowing the characteristics of how that steam trap operates, you can make a judgment to see if it’s faulty or not.”
Most facilities are far from silent, and it can be vital to filter out extraneous noise which masks the relevant sounds: “Sometimes in power stations you can’t filter out turbine noise, and it appears that the steam trap is passing,” says Cull. “That’s where the thermal camera would show you the upstream and downstream temperatures and whether that trap is passing steam.”
Michael Povey uses a TLV system “which measures the ultrasonic noise going through the trap, and the temperature at the same time. The noise is recorded and compared with a database of trap units – then it says this trap is leaking this amount of steam per hour. And we check upstream noise; if it’s beyond certain tolerances, the system says you have noise elsewhere. We take another recording and deduct one from the other.”
“We do everywhere from laundries and bakeries at the small end – 30-40 traps, maybe – through chemical plants to oil refineries. We’ve just done a refinery with 6,000 traps on it, and it had a 40% failure rate!” At this scale, the plant could be losing millions of pounds a year to failed traps.
Povey also cites a recent survey of 35 traps: “Two had failed closed and four had failed open, costing £1,200 a year each. Replacement traps for that site would cost £3,000 – and after ten years would typically show just a 7% failure rate.”
CAUSES OF FAILURE
“Typically 5-10% of steam traps may fail a year,” says Andy Cull, “but that’s a very rough guide. We were at a power station recently which had 44 traps which had been running for over 18 months and none had failed.”
Reasons for trap failure include a build-up of additives intended to keep the pipework clean. Cull adds that water sources are a factor: “RO [reverse osmosis] water is pure, so there’s no debris going through. If you’re pulling borehole water, and it’s got a lot of suspended solids in, that can be a problem.”
The issue may not be trap failure, says Michael Povey: “A lot of steam systems were poorly designed, or were correctly designed but have had an extension built on to the factory, so you get steam starvation. Or they have made part of the plant redundant, so a steam pipe is oversized, and you have unnecessary heat loss. It’s cheaper to change the pipe than to live with the heat loss over a number of years.”
“Some traps cost more to purchase, but will last 10 years,” adds Povey. “Cheaper traps may have a 2-3 year life.” And selection of the right replacement is critical: “Just because it was in there doesn’t mean it was the right trap. Steam traps are treated as commodity items, but they are an integral part of the system.”
BOX: SPIRAX SARCO WIRELESS MONITORING SYSTEM
Spirax Sarco’s STAPS wireless monitoring system is a self-contained unit which clamps on to a pipe next to a steam trap to evaluate its function. STAPS can diagnose failed-open traps which are leaking live steam, and those that have failed closed or are blocked. It transmits data via the ISA100 wireless standard, and has a hot-swappable battery with a three-year life.
Include dirt pockets, so the traps are not collecting dirt from the pipelineA strainer must be fitted, integrated into the trap or before the trap, to protect the mechanismRegularly check steam traps – have a programme of trap maintenance.