Thermal expansion relieve valves 2

[peter Hond]

Dear all

Although API 521 suggests that a nominal diameter of (DN) 20 × DN 25(NPS 3/4 × NPS 1) relief valve is commonly used for only thermal relieve scenarios because oversizing in this case is fine, I would like to calculate the 'exact' relieving flow and DP's for a thermal expansion PSV in our plant.

There are 3 cases around one location that I want to calculate the PSV. One thermal expansion PSV is at the suction side of a reflux pump, the second one is at the pressure side of the same reflux pump and the last thermal expansion PSV is used for a blocked pipeline.

We mainly use Aspen Plus/safety analysis for these calculations. Aspen and also the API 521 requires the heat transfer rate. Can anyone please help me in determining the heat transfer rate for thermal expansion.

Thanks in advance!

 

[pierreick]

Hi,
Consider the document attached to support your work.
Good luck.
Pierre

 

[The Obturator]

What you are asking for is the 'Rated Capacity' of your Thermal-relief Valve.

All you need to do is to use the formula in API-520 (2020) section 5.8. Here you will find the sizing calculation to determine calculated capacity of the valve. Simply transpose the formula to calculate the flow. To do this you will need:-
Set Pressure
Back Pressure (total)
SG
Viscosity (if high)
Co Efficient of Discharge (or use the default value listed in not known)
Orifice area of the TRV
(the last 2 items are dependent on the TRV manufacturer - their catalogue should state them - and also the flow calculation method).

I will add that you also need to understand if the TRV in question has its capacity certified to ASME VIII or not - depending which will determine which formula you use - more is explained in the section of API-520 (5.8 - certified / 5.9 non certified).


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[peter Hond]

Dear Pierreick

Wong does indeed calculate the relieving volume. But how to further calculate the relieving flow in kg/hr?

 

[LittleInch]

The heat transfer rate depends on many things.

Differential temperature from outside to inside, buried or A/G
Wind velocity
Surface area
Insulation
Other forms of heat other than solar or ambient air temp

So you need to calculate U value for your pipes. A figure between 10- 20 w/m2/K seems to be a good range.

The heat input for a thermal relief system of a static locked in section though will vary over time as the contents warm up and expand and will eventually reduce to near zero as the temperature outside to inside equalises.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[pierreick]

Hi,
Consider the document attached, pages 32-34
Pierre

 

[georgeverghese]

Where do these TRV's discharge to, and is the max backpressure at the disposal side less than 10% of TRV setpoint in all 3 TRVs'?

 

[Snickster]

Here is a specification procedure used by a client located in saudi arabia. The solar radiation default value may be a little high but you can always substitute with solar radiation values of your actual site with such as ones provided in ASHRAE.

4.2 Unless a more rigorous analysis is made, thermal expansion calculations shall be
based on conservative assumptions including the following:
4.2.1 The pipe section shall be assumed to be blocked in under the following
conditions:
a) while at the ambient temperature or at the normal operating
temperature whichever is lower;
b) while at highest expected pressure.
4.2.2 Solar radiation of 950 W/m² (300 Btu/hr.ft²) shall be assumed during 10
hours on the projected area of the exposed pipe surface.
4.2.3 Any temperature rise of buried or insulated pipelines which normally
operate at or above ambient temperature may be disregarded. The
temperature of refrigerated liquids in buried or insulated lines shall be
assumed to rise to the ambient after being blocked in at normal operating
temperature.
4.2.4 The heat loss of the exposed pipe surface to ambient per unit length of
the pipe shall be determined in accordance with equations (1) and (2):
By convection:
Qc = C1 * D**0.75 * (T1-Ta)[sup]1.25[/sup] Watts(Btu/hr) (1)
By radiation:
Qr = C2 * D * [(T1/100)[sup]4[/sup] - (Ta/100)[sup]4[/sup]] Watts(Btu/hr) (2)
where:
C1 = 1.28 for SI units and C1 = 0.85 for customary units
C2 = 8.97 for SI units and C2 = 0.27 for customary units
D = pipe outside diameter in meter or feet
T1 = pipe temperature, degrees Kelvin (Rankin)
Ta = ambient temperature, degrees K (R)

 

[peter Hond]

Dear Pierrick, Snickster

Thanks a lot, both cases will give me a good calcuation method!

 

[peter Hond]

Dear georgeverghese

All these PSV's goes to a Absorber column.
Before i'm determining the plosses, I first need to calculate the scenario relief load.

 

[georgeverghese]

At zero to low wind velocity, surface temp of aluminum jacket for equipment and lines wrapped in cold insulation is roughly 55-60degC max.