TREVITESTS of safety valves 1

[athomas236]

We are supervising the construction of some HRSGs in the Middle East and a question has been raised about whether the boiler safety valves should be "floated" in the traditional way to confirm opening and closing pressures or whether the valves will be tested using TREVITEST equipment.

Does anyone have experience with this type of equipment and whether it is generally acepted alternative to "floating" valves.


athomas236

 

[yogibear1]

I have used this method (didn't know it by that name) a number of times to successfully set safety valves. We had the safety valve rep (from Desser) do it. It is easier and quicker. It also avoids the actual pop off of a valve that may cause marring of the polished seating surfaces.

 

[alexander123]

What is meant by "floated"? Is it just the lifting of the lever?

We use a trevitest method all the time with much success.

 

[athomas236]

alexander123,

Floated means increasing the boiler pressure until the valves lifts.


athomas236

 

[alexander123]

athomas236,

Doesn't that seem a little scary? What if the valves don't lift?

 

[athomas236]

Thats why we do it carefully and have a guys standing next to the pressure gauges by the safety valves ready to instruct the operators to shut down if there are problems.

athomas236

 

[alexander123]

ahhh.....it's good when things are done carefully.

good luck

 

[Sideburns]

Good question athomas236.

I would be very interested to hear as well if it could be accepted by the 3rd party inspectors in EU (like TUV)? Does this method really pass as a "traditional pop-up test"?

/Frosty

 

[Biggy]

Hi,

On our site we use Furmanite to Treviest our valve on our steam mains every year, our Inspector is happy with this certification. This is for 50psi reduced from 210psi.

It is still scary, but not as scary as actually lifting the valves by floating them.

However he does insist on floating the valves on the boilers every year. As stated very scary, once when theylift and the most scary is waiting for them to reseat, bearing in mind you are loosing water faster than the pump can keep up, so you can get to a situation where there is no water left. Ouchy!!

Biggy

 

[sjrfc2]

We looked into this and used it before. It can give you a set pressure but you have to have a good knowledge (drawings) of the safety valve. It works by applying a measured force and calculating a force balance when movement of the stem occurs. You have to know what the disc seating area is so you know the area the line pressure is at. Furmanite has info on standard valves. They acually use mean seating area calculated as the mid-point between the seating inner diameter and outer diameter (rather than at inner diameter where fluid acts) because of inconsistencies in the seal.

ASME has looked at this method and a discussion of it is in Section VII as well as PTC 25 and the OM Code. I think its referred to as a hydraulic lift assist device or something. My understanding is that if a contractor has a VR stamp and uses one of these devices then they've demonstated accuracy is compliant with the PTC code.

There are limitations:

1. The valve design has to have an exposed and threaded spindle/stem that permits mounting the device and exerting a load (need space over the valve as well).

2. Its only used on compressible fluid. NRC bulletins noted a problem with this device when used on a valve in a loop seal. I don't remember whether it was a thermal difference problem or a problem with liquid rather than steam on the valve disc.

3. Not good for valves smaller than 1".

4. NRC also noted a problem with acuracy of testing a specific type of valve (with a thermal lip or thermal seal or something). I remember that it had a very wide saeting surface and thinking that that may have impacted the mean seat area calc and where the point of sealing really was.

The device provides only setpoint. You need to be operating and I think they target within a certain percentage of setpoint for good results. It will not provide info on blowdown but I came away from the Codes reading that you were OK to put the blowdown ring back to OEM or test settings (didn't have to provide blowdown demonstartion).

Other folks besides Dresser and Furmanite do this type of testing. Atlantic Group does it too.

What seemed to be really nice about this device is that it allowed us to have valves worked in place when there was no pressure on the system rather than removing them and sending them out (repairs done by removal of bonnet). Downside of that was that you needed the contractors there when the plant was being brought up to set the valves (and the schedule always moved resulting in them standing around on your dime).

Another advantage seems to be that you are not really popping the valve (in the ones I've seen) and you are limiting the damage donne during lifting and reseating. This seems like it buys you time between repairs. One of the NRC Info notices or bulletins talked about the endless cycle of reseating the valve, popping it and them repairing again because it leaked after testing.

 

[sjrfc2]

Sorry, One more thing, Where valves are removed and set in a shop or lab, they can be "off" when they are re-installed in the plant due to changes in ambient temps. Some of the NRC info I read through documented as much as a 4-8% change in setpoint because the valves were set in a lab where the ambient temp was cooler.

 

[athomas236]

sjrfc2,

My understanding of ASME I is that both the set pressure and blowdown have to be demonstrated. I do not think that trevitesting or similar techniques do this and therefore do not comply with ASME I foe new plant.

I have seen a copy of the ASME approval letter from years ago that says just this.

However, for periodic testing once the boiler is in service, I can see the benefits of trevitesting or similar as a way of confirming set pressures.

I understand the need to know the seat area, but it becomes less important the closer the operating pressure is to the set pressure.

I agree with biggy comments about water losses.


Best regards,

athomas236

 

[sjrfc2]

Thanks, I was just speaking from in-service testing.

For the seat area, I still think you need an accurate number or your not going to get accurate results. When we were faced with testing a few valves and we didn't have good info on seat areas, it was proposed that we test them using the lift device at varied line pressures. This would provide several data points and then you could "back out" the mean seat area based on the different force required at different line pressures. This adds variables to the accuracy and I'm not sure it was ever tried but I thought it was an interesting approach. Unless you had a valve that was lifting consistently I'm not sure what type of accuracy you'd get trying to back out a mean seat area calc. But if you didn't have the valve data (the contractors probably will have it if its a standard valve) then its something to at least try.