Floating boiler Safety valves after an overhaul - query

[robsalv]

In Australia, when a boiler has been down for an inspection or overhaul, or for a safety relief valve overhaul, it's standard practice to do a controlled lift of the safety valve [boiler firing against the blocked stop valve with superheater vents open to keep some flow]. We record lift and reseat pressures. We try not to lift and or adjust the SV settings more than 3 times, otherwise the SV will leak on line.

There is a move by occupational health and safety folk to stop this practice because of the noise involved. In my experience, trevi testing is not a total answer - so I'm on the side of floating SV's.

Is this "floating" a common Australian practice? If not, has anyone gone from floating to not floating when recommissioning their boilers?

Thanks

Rob

 

[spector]

Around here we send our SV's out to a test/repair concern every year(by law). There they are inspected, tested and repaired as necessary. The valve is then sealed and tagged showing date and capacity. If the seal is later found broken, the valve is then invalidated and must be recertified.

While installed and in service, a Boiler Inspector will sometimes require that the boiler undergo a capacity test like you described. Such a test is used when total valve relief capacity is suspect. Or when installation configuration might restrict or hinder proper valve operation including valve reseating.

If you elect to not op test your valves, but instead send them out for test, then you need to take a close look at how the valves are installed. Ensure escape piping is properly configured. Do not allow escape piping to place loads or stresses on the valve body or it may effect valve operation. Provide plenty of capacity beyond output calculations.

You should consider locating a SV repair/retest concern to recertify your valves vice testing them in operation if noise is a problem.

 

[robsalv]

Thanks for your reply Spector.

You're totally right. An offsite overhaul by a mob with a test rig is the answer. Obviously, you're satisfied with the capacity test tagging but are the valves being tested on steam or air?

I'm only aware of two mobs in Australia that have a capacity test rig, however both operate on air. To my mind, air is not steam.

I've looked at these mobs and have not been convinced that their correction factors are sufficient. In our case, we operate right on 95% of set [very tight] and small margins of error mean a big difference to valve life. The other thing is that the mobs are over 1000 kms away and I'm mindful that the valves are likely to take some knocks and vibration during transportation - quite possibly destroying all the good overhauling effort. Unfortunately, offsite overhauling does not seem to be the answer in our case.

 

[spector]

I'm lucky in that I live less than 10 miles from our nearest SV shop. They do indeed use steam to perform all tests on boiler safetys. They are also set up for other test mediums. The shop must undergo a yearly recert by the State Boiler Inspector. He not only inspects their boiler, he also reviews their complete operation.

I also live 3 miles from a Dresser Valve plant where they operate 6 Vapor steam generators. One of their boilers operate at 2250 PSI. The boilers are used to test their steam SV's and other products. I can easily hear their tests from 3 miles distant. It's never a problem unless they work a third shift.

You present testing is certainly the best way to go, but if you must stop op testing, then you really have no alternative but to send them off.

 

[mur]

Your present testing regime is definately the best practice, and a generally requirement in NZ. lots of experence of PSV's, RV's coming back from "specialist" shops
and lifting light.

 

[aybee]

If a boiler psv has been tested and re-set in the shop it MUST be floated ON THE BOILER before it is put back into service..Australian statutory requirement cant remember if its in AS3788 or AS1228. Trevitest is no good for this because the steam test is a lift and blow down test to ensure that it drops the pressure by passing a set volume . Trevitest will check the pop pressure but wont be any good for the blowdown rate...there is a setpercentage of the boiler steaming pressure which has to drop before the valve re-seats (3% I think)
Yes its common practice and again its mandatory to do it at least on your boiler interlock check maximum period which is 2 years in Australia.( and any other time the psv gets worked on ) The environmental watchdogs wont let you do it at night , but they cant stop you doing it

 

[JAlton]

In Line Testing of Pressure Relief Valves is very seldom used in the USA over the past 20 years. However, it is still used in Suriname. It is the Standard to which all other Test Methods (Test Bench, Auxiliary Lift Device, i.e. Trevitest) aspire. If you decide to use a PRV Repair facility, keep in mind that not only are differences between Air & Steam Testing, but the Accuracy of Bench Testing on Steam is also affected by Warm-Up Time on the Test Bench. If a PRV Spring is not sufficiently heat soaked prior to final testing, the Set Pressure will drop off once the PRV is installed. Also, depending on the size of the Boiler, the Test Bench Accumulator will probably NOT be sufficient to set actual blowdown (reseat pressure) The ASME Code requirement is 4%, but there are some highly engineered Boiler Safety Valves capable of delivering full flow at 3% blowdown. If the Accumulator is of insufficient size, the PRV blowdown is a function of Vessel Size (all steam is evacuated from the vessel on pop) rather than a function of PRV Ring Adjustments.

One path forward to consider, provided jurisdictional authorities are agreeable, is to perform the repairs yourself, or contract to have them repaired, in place using the following scenario:

Step 1: Perform Live Steam Testing on the Boiler prior to a scheduled outage (Maintenance shutdown). Make any necessary Adjustments to Set Pressure and Blowdown.

Step 2: Perform Repairs during Shutdown. Be careful to record all adjustments during disassembly in order to return the PRV to its "as found" adjustment.

Step 3: Following Repairs, when the unit is back on line for at least 24 hours (Heat Soak for Springs), Test the PRVs using Trevitest or another Auxiliary Lift Device to reestablish (confirm) set pressure.

The Logic of this method is the Blowdown should not change if the Adjustments are returned to the proven settings from before the shutdown. Then perhaps once every five years one or more PRVs could be live tested to demonstrate the accuracy of the set pressure & blowdown settings.

J. Alton Cox
President
DeLuca Test Equipment

http://www.delucatest.com