Temperature Relief on Steam 1

[TMEldridge]

I have what may be an easy questions...

I am designing a system involving a steam-water heat exchanger. The steam header is superheated steam (250psig, 600degF), and there is a pressure reduction station (2 PRV and desuperheater) between the header and the heat exchanger passing up to 74,000 lb/hr.

Problem is I need to design for failure of any one component, which means a pressure relief and some type of compensation in case of desuperheter failure. The end product will be steam at 60 psig and 375 deg F. The heat exchanger is rated for this, but not the ~550 deg F if no desuperheating takes place.

How do I assure that the still-superheated steam will not be able to enter the system in case of desuperheat failure? T&P valves seem residential/commercial only.

Thought?

 

[davefitz]

If you are designing the system, then it would seem you have the flexibility to provide components suitable for the casualty case of desuperheater failure. Carbon steel works fine up to 750 F , just include the 550 F as a design max temp in the spec for the equipment.

Other casualty cases are:
a) PRV fails open and overpressurizes the 60 psig header; this requires a safety rfelief valve sized for the casualty flow from the 250 psig system

b)tube failure: if teh water is pressureized above 60 psig, thena ruptured tube can overpressure the 60 psig header as well, so the relief valve should also be suitable for the flow from a burst tube.

 

[TMEldridge]

True that I can choose whatever I need, but budget constraints restrict the "custom" heat exchanger required for 150psig and min 550 deg F. The custom cost is far more than the cost of a relief valve or (hopefully) another scenario. The manufacturer design rating of 150psig at 375degF is off-the-shelf. A pressure relief is planned for this application, but does not remove the temperature concern.

 

[davefitz]

You may ask the vendor for what is the derated pressure rating at 550F. If it is more than 10% over your 60 psig setpoint header pressure, you might be able to use the low cost off the shelf item at a derated pressure.

 

[psafety]

eldridge9,
Look at what could fail with desuperheating. Basic question: is the desuperheating water flow measured ? and does the steam have flow control (on/off)?

Don Coffman

 

[abeltio]

is this equipment built under a code or spec?
you may have the answer there, but it may say that the equipment must be built to sustain the max press/temp possible in the system.

to avoid the steam from entering the HX in case of failure there are valves that operate on the temperature of the fluid... you can get a valve that closes if the steam temp reaches 350degF which gives you 25F... again this system may in itself fail ... so the construction code governs again...

if it is possible that the PRV's and the de-superheating fail at the same time then the HX should be designed for the worst case scenario regarding steam conditions... a custom built HX might be expensive... but never as expensive as a lawsuit.
HTH

saludos.
a.

 

[rmw]

You are going to have to put in a device to sense downstream (of the DSH) temperature, and when and if the DSH fails to functions, then you can make up your mind what you want done. Do you want the air supply to the PRV's dumped so as to quit letting down superheated steam, or do you want to open a valve to atmosphere, or some other steam header???? Your temperature controller for the DSH might have alarm functions that could close switches, etc., that could provide a protecting action for you.

You are dealing with two distinct problems. One is PRV failure, and safety valves do that work. The other is DSH failure, and your indication is excessive temperature, so you have to determine what to do to protect the Hx. Block flow, or dump flow.

Or, did I understand your question adequately??

rmw

 

[tickle]

Have you spoken to Spiax Sarco (or similar) do they have an off the shelf product for temperature limitation and protection?

 

[TMEldridge]

I think I've gotten it. Thanks for all your help.

I'm keeping the normal pressure relief valve for PRV failure and adding a temp switch downstream of the desuperheater which will close the PRV pilot line and force the valve shut.

 

[TD2K]

I'll just add to some of the other comments that I would strongly suggest to try and find a design for the exchanger (such as dual pressure/temperature rating) to see if you can make the system 'inherently' safe.

A lot of the companies I've done work for would not accept a single temperature switch as adequate protection against exposing a pressure vessel to excessive temperatures.

It works on paper but if the switch isn't working in the field, you've lost your protection. You also need to ensure the switch is put on some sort of preventative maintenance schedule AND what that schedule is which is a decision based on an evaluation how often the switch is likely to fail, how often is the desuperheater likely to fail and the resulting risk to the pressure vessel.

Bottom line is: you don't want a loss of containment if this happens. What is the cost IF the vessel is exposed to maximum steam temperatures? Do you likely just have to inspect it? Would it have to be replaced? That may suggest another means or protection or a redundant protection system (two transmitters for example, one for indication and one for shutdown so you can see both are working).