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# Thermal Relief For Exchanger On A Blocked-In Condition

2 replies to this topic

### #1 PinoyCan

PinoyCan

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Posted 27 June 2012 - 08:05 AM

Hi Guys,

I am checking if a thermal relief valve is required on the cold side of a new exchanger in case of the blocked-in condition. Isolating block valves are to be installed on the inlet & outlet lines of the cold stream to/from the exchmage. The following are the operating conditions:

Hot Side:

Fluid: Inhibited Water
Inlet Temperature: 85 C
Outlet Temperature: 33 C
Inlet Pressure: 370 Kpag
Design Pressure: 1,380 Kpag

Cold Side:

Fluid:Recycle Water
Inlet Temperature: 22 C
Outlet Temperature: 79 C
Inlet Pressure: 720 KPag
Design Pressure: 1,448 KPag

The design pressures are based on the maximum shut-off pressure of the pumps.

My analysis is that, a thermal relief valve is not necessary because, for the cold-side pressure to reach the design pressure of 1,448 KPag during blocked-in condition, the Recycle Water must attain a temperature of 200 C. Since the Inhibited Water temperature (heating medium) is only 85 C, it is not possible for the Recycle Water to have a temperature of 200 C.

Is my analysis proper? Please comment.

Thanks

### #2 latexman

latexman

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Posted 27 June 2012 - 08:43 AM

To get an idea of the pressure developed on heating a packed liquid you may divide the absolute values of the cubic thermal expansion of the fluid

α = (1/ V)(∂ V/∂T)p

by its isothermal compressibility

κ = -(1/ V) (∂ V/∂ p)T

for the envisaged temperature range.

α and κ can be estimated from data from reliable references, like http://webbook.nist.gov ; α from the density changes, and κ from the sound speed in the liquid.

I have two data points for water in my company design manual that bracket your application: 320 kPa/oC (20oC) and 1280 kPa/oC (100oC). If your cold side is blocked in at 22oC and then the hot side flow is started, with only 2-3oC of temperature rise, you may have a problem!

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Posted 27 June 2012 - 10:26 PM

The requirement for addition of Relief valve on the exchanger has to be analysed for others relief scenario scenarios narratives and than the most governing scenario with its load is used for sizing the relief valve, I presume that you analysed for other cases also like Tube rupture, Fire, Blocked in and in this case you are trying to estimate relief load for one Bocked in Thermal expansion case. For this first you have to estimate Coefficient of cubical thermal expansion (tha can be estimated from specific volumes at normal operating pressure & Relieving pressure) and than you can use standard formulae for estimating load for the relief valve.