We have a safety relief valve on ou

[ProjectEng]

We have a safety relief valve on our 175psi steam header. The discharge of the relief valve is piped through the building and up through the roof. A drain leg w/ temp probe and control valve is piped to the floor.

I assume whoever installed this setup thought that as condensate cooled below the 175psi steam temperature, the control valve would open and allow it to drain to the floor. And if the relief valve would lift, the higher temperature would trigger the valve to go closed.

It seems to me that this setup can all be simplified with a steam trap. Any thoughts?

 

[MJCronin]

Projeng,

It sounds like the control valve can be replaced bya thermostatic steam trap.

Is the collection of condensate on the outlet side of the relief valve ? If so, the condensate would be at atmospheric pressure and I would see no need even for a trap. (a periododic drainage of the collected condensate should suffice)

If the condensate collects on the inlet (HP)side of the trap, ensure that any new steam trap is sized for steam system start-up. Condensate loads under start-up can be many times the normal flow

My thoughts only...................................

MJC

 

[ProjectEng]

Well a ball valve won't suffice because it will either get left closed too long (discharge side of relief valve will fill up with condensate) or it will be left cracked open (relief valve lifts and sends high pressure steam through ball valve instead of roof vent).

I've never seen a temperature control valve on this type of application but I think a steam trap is perfect for it. It would drain any condensate on the drain leg and high pressure steam would still be forced up through the vent.

 

[saxon]

Proj.Eng., Yes, a steam trap could be used on the drip leg, but this may be a situation where the temp. sensor and switch tied to the control valve may have been set up to give a visual/audible indication when the Header pressure has exceeded the Header RV set point pressure (based on control system dead time) and the blowing of live steam until the control circuit responds. As you have said, the relief is to atmos. beyond the roof line, out of everyones sight. This would be a bit of an odd alarm system.

It may also be, that this control circuit was set up to guarantee that the discharge side of the RV is free draining, normally must traps require some motive energy to operate correctly and when the RV closes you lose that energy.

Just my two cents worth.
saxon

 

[ihg]

There is a common requirement in boiler codes to have an indication at the boiler floor that a PSV has lifted, and this is done by having a drain piped into a tundish from the PSV discharge - rude, crude and mechanical but it used to work when every boiler had a boiler attendant. It was also dangerous to the boiler attendant. With automation, someone has got a bit over tricksy - the temperature element to an alarm would be adequate, and a thermostatic trap set around 105 C would avoid blowing steam while allowing the vent pipe to drain rainwater and residual condensate. The vent pipe will be running somewhere above 15 psig if the valve lifts and you have sonic flow in the tailpipe.

 

[ProjectEng]

I'm still undecided whether or not the steam trap would work without any motive force. In the normal condition, the PSV is seated but leaking by a little. This small amount of condensate needs to be drained out but the drain leg needs to close when the PSV lifts.

saxon seems to indicate that 'most' steam traps don't work without some motive force while ihg says the steam trap on the drain leg would work even while the PSV is seated.

ProjectEng

 

[TD2K]

If you put the steam trap at the bottom of the drip leg off the main discharge header, the condensate would build up until it had enough head to flow through the trap to sewer.

When the PSV relieved, the hot steam would cause the trap to close. I'd look at something like an inverted bucket, a thermostatic trap would allow some steam to blow through the open trap I'm thinking before it would respond to the rising temperature and close off, I'm just not sure how fast those things respond.

 

[hacksaw]

I agree with TDK, there are quite a few float types that can easily shut-off against the back pressure in the vent pie (during full lift conditions).

Usually the drain is just poiped to a drain. If you have a high capacity relef valve, you need to check the built up back-pressure. It can be significant.

 

[MJCronin]

To all,

TD2K is correct in stating that an inverted bucket trap is another reasonable choice for this service.

A quick discussion with a technical rep from Armstrong Int'l or Spirax Sarco would confirm the trap choice.

Saxon states "....normally must traps require some motive energy to operate correctly...." Yes, that is true, and the difference in system pressures make these devices reliable, simple, cheap and in use all over the world......why would you want to try anything else ?

My opinion only........

MJC

 

[mur]

Check your code. Many would find your setup illegal having any valves/traps on the drain line. If these fail for any reason and with the potential for liquid buildup in the discharge line imposing excessive back pressure on the PSV.

This whole setup scares me.

 

[ProjectEng]

If you have no valves/traps on the drain line, how do you keep steam from exhausting through the drain line instead of the vent silencer?