7 replies to this topic

[aniljani]

Hi friends,

We have a gas heater, where the gas at 0.5 barg is being heated by Medium Pressure Steam at 24 barg.
Steam is passing through a steam coil and gas is passing through a chamber surrounding the coil?

Chamber is designed for 1 barg pressure.

Whether should we consider pin hole leak or coil rupture (traditional way of assuming 2 equal orifices)?

How to esimate relief rate?

[fallah]

aniljani,

Based on equations in Crane TP No. 410, you can use following equation for steam relief rate in coil tube rupture:

W=1,444.6Av(1-0.317 dP/P1)(dP/LOv)^0.5

Where (considering you have critical flow condition):
W=Steam flowrate, lb./hr.
Av=Coil tube diameter, Sq. inches
dP=P1-Pcf (P1=Upstream relieving pressure, Pcf=Critical flow pressure, both in psia)
LOv=Steam density, lb/cubic ft

Fallah

Edited by fallah, 12 March 2012 - 04:55 AM.

[shivaprasad]

fallah,

When we need to consider small tube leak or coil rupture as per API 521?

Shiva

[fallah]

Shiva,

When the pressure in low pressure side (coil or chamber) during coil rupture doesn't exceed the relevant corrected hydrotest pressure, it don't need to be protected by pressure relief device.

Fallah

[shivaprasad]

Fallah,


In the case of shell&tube heat exchangers, as per API 10/13 Rule on exchanger side subject to tube rupture, whether the low pressure side is on the shell side or tube side. The ratio of low pressure side design preesure to high pressure side design pressure is less than 0.77 (10/13) of the high pressure side design pressure (per API STD 521, section 5.19.2), where tube rupture is applicable.

But when we need to consider pinhole leak or complete tube rupture for shall & tube heat excahgers?

Shiva

[fallah]

<sub>Shiva,

In last ver. of API 521 (Fifth Ed.) you cannot find anymore about 10/13 rule for heat transfer equipment failure. In fact, the criteria is such that when the pressure in the low pressure side (could be tube side or shell side) during tube rupture doesn't exceed the relevant corrected hydrotest pressure don't need to be protected by pressure relief device against tube rupture's overpressure.

For calculating the required relief load due to tube rupture, as per API Section 5.19.3 two orifices may be used as calculation basis.

Fallah</sub>

Edited by fallah, 02 August 2012 - 12:18 AM.

[Raj Mehta]

Can somebody provide drawings/print screens explaining the same? I am making drawings of the scenario's but Its a bit difficult for me to visualize it. Also I dont want to build my foundation on wrong assumptions or notions.

for eg. low pressure on shell side & then what would happen with low pressure on tube side. ? How exactly the rupture would occur (driving force) ? How are the pressure relief devices placed on the heat exchnager ?

I would really appreciate your help.

[Olidin]

But when we need to consider pinhole leak or complete tube rupture for shall & tube heat excahgers?

 

We just recently had this question and I thought I'll chime in this very old post. 

 

We chose to consider complete tube rupture if the tubes are thin, probably very small tubes inside the TEMA type exchangers. These can completely break in half. A 2 orifice method is appropriate here. 

 

Now, sometimes we have large coil that pass steam through the tubes. But these tubes are not inside an exchanger and they are actually piping, in 2" to 10" diameter. Probably schedule 40. Considering that these "tubes" would break cleanly in half without an prior sign of wear or leaks is very very very unreasonable. In this case, it is more appropriate to assume a pin hole leak. How big should the hole be? well, that is up to debate as well. Use your judgement.