TSV and PSV

V

Thread Starter

v venkatapathy

Hi List Members,

Can anybody help me out to understand the difference between a Thermal
relief Valve(TSV) and a pressure safety valve(PSV), Esp. How they decide
upon the set pressure and basis of selection of the above two types of
Valves i.e for which applicaitons these valves are used.

Thanx in Advance,

WIth Regards,
Venkat.

 
C
Venkat:

First let me preface my response by saying that the sizing and application of safety relief devices is something that should be done only by a qualified individual.

Typicaly a thermal relief valve (TSV as you call it) is a low flow relief valve used on equipment protect against over pressurization due to thermal expansion. A pressure relief valve (PSV as you call it) is used for relieving larger volumes due to process of external hazards. A hazard analysis around the equipment to be protected must be performed in order to determine what the safety device must be.

Two examples may make it clearer.

Case #1

Shell and tube heat exchanger.
Shell side MAP 150 psi, stock on shell
Tube side MAP 150 psi, steam in tubes
Maximum shell side pressure 90 psi (determined by pump size, nitrogen line blow stab ins, etc.)
Maximum tube side pressure 150 psi (determined by safeties on boiler for example)
Hazard analysis shows isolation valves on the inlet and outlet of this exchanger, the potential exists for the shell side to be full of stock when isolated and steam to be in the tubes. The shell must be protected for thermal expansion.

Case #2

Shell and tube heat exchanger.
Shell side MAP 150 psi, stock on shell
Tube side MAP 900 psi, steam in tubes
Maximum shell side pressure 90 psi (determined by pump size, nitrogen line blow stab ins, etc.)
Maximum tube side pressure 800 psi (determined by safeties on boiler for example)
Hazard analysis shows isolation valves on the inlet and outlet of this exchanger, the potential exists for the shell side to be full of stock when isolated and steam to be in the tubes as before, but there also exists the possibility of a tube rupture causing the shell side to be exposed to 800 psi. The shell side would need a pressure relief valve or rupture disk to protect it. This would need to be sized to handle the full stock flow and steam flow.

The hazard analysis is the key to determining what is needed.

I hope this helps.
 
B

Bruce Durdle

Both are in fact pressure-operated.

A thermal relief valve is designed to allow a small flow of liquid to
escape from a pipeline. Typically, one will be used on every section of
line beween valves that is exposed to heat - maybe solar if the line is
above ground.

A pressure safety valve is designed to pass large quantities of liquid to
avoid overpressurising. It will be used where there is a possibility of
large volume flows of liquid being forced into a vessel when the discharge
cannot handle them, or where failure of a pressure control loop would allow
pressures to increase above the MAWP.

The set pressure will be laid down by pressure vessel codes applicable to
the plant. In general, the releif valve will be set to the Maximum
Allowable Working Pressure for the vessel or pipework being protected. But
check the codes that apply to the design of your plant.

Bruce.
 
D
Venkat,

I am assuming you are referring to self-contained PCV (pressure control valve) and TCV (temperature control valve). These are merely regulators that
incorporate sensing, controller and valve into one unit. Compared to a control valve, they are smaller in size, cost less, do not need air supply and require no remote control as set point is adjusted at regulator. The adjuster is set to a predetermined pressure and/or temperature whichever may be the case. Sensing is either against a diaphragm (pressure) or bulb (temperature).

David Baird
Senior Controls Engineer
GEM CITY ENGINEERING
DAYTON,OH 45404
(937)-223-4600 x1456 FAX (937) 226-1908
[email protected]
 
V

v venkatapathy

Mr. Baird,

Infact, I am talking about Thermal relief valves and Pressure safety valves and not about self actuated valves. My doubt is, for these type of valves, what is the basis for calculation of set pressure (for both type). Do thermal relief valves has anything thing to do with temperature limits??????. Do They really decide upon the set pressure for thermal relief based on temperature.?????. Any specific applications where thermal relief valves are used other than Cooling water lines in exchangers.

Thanx.
Venkat.
 
N

Nilanjan Mukhopadhyay

Hello Venkat
I believe you are looking for som information about the Normal Pressure Relief /Safety Valves (PSV) and the Thermal Relief Valves (TSV). The TSVs are normally very small in size like 3/4'' x 1" . This only relieves the over pressure created by ambinet temperature swing on water or similar liquid circuits . The normal overpressure is 25%. The PSV s on the otherhand are guided fully by API standards . Their overpressure is 10 % or 20 % based on sizing basis like a block discharge or External fire case .
Nilanjan Mukhopadhyay
Sr Instrumentation Engineer
Haldia Petrochemicals , India
 
dear Sirs
Normally relief valves are for liquid services.
Safety valves are for gas/vapour/steam applications. Safety relief valves can be used for both liquid and gaseous application. Now let us come to the point. thermal relief valves are for liquid sevice ONLY. In a line,for example cooling tower line,there may be a slight increase in volume due to temperature increase.This sudden increase of volume in flow has to go out.otherwise the pipe may go off.When volume is increased the pressure will also increase.
To avode this a Thermal relief valve is used.Since the increase of volume may be in terms of drops.capacity calculation need not be done and can be a small sized one say 1/2 inch or 3/4inch.
M S Rajan B.E.,M.B.A.,
Asian industrial valves and instruments
B17,Industrial area
Mogappair East
Chenni-600050.India
[email protected]
[email protected]
 
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