13 replies to this topic

[go-fish]

Hello

I am trying to determine the design pressure of shell and tube side of a shell and tube heat exchanger. Both services on the shell side and the tube side are pumped liquids.

Does that mean the design pressure on both shell and tube side will be the pump shut-off pressure of the corresponding centrifugal pumps?

Thanks

[fallah]

go-fish,

Provided that there would be isolation valves in tube side/shell side outlets, the design pressure of tube side /shell side would be relevant pump shut-off pressure plus or minus (depending on piping configuration) the pressure due to the static head between the pump and the exchanger.

[S.AHMAD]

You can choose either use the maximum possible pressure or lower pressure with PSV. lower pressure option is normally justifiable for exchangers installed downstream of a control valve. If you show the PFD, probably we can give a better comment.

Edited by S.AHMAD, 11 January 2013 - 05:03 AM.

[go-fish]

I have attached a schematic flow diagram. I would appreciate if you could advise on the design pressures with break points along with the approach for PSVs.

We are not controlling the flow and temperature of the services as the main idea is just using the hot fluid for heating the colder fluid.

Attached Files

•  flow diagram.PDF   25.49KB   276 downloads

[fallah]

go-fish,

Appears the isolation valve in the outlet line from shell side is forgotten to be considered. Anyway assuming fluid in both sides to be water, using the information on the sketch the design pressure of the shell/tube sides would be around 4.5/9.5 barg. Tube rupture case for this exchanger appears to be a credible scenario and should be evaluated among all credible scenarios. Nothing specified about the fluids in both sides and it isn't clear which side handles the hot fluid. Then to specify the needed PSV, at first all credible scenarios should be determined and quantitatively evaluated.

Edited by fallah, 11 January 2013 - 03:36 PM.

[go-fish]

The hot fluid is on the shell side.

[S.AHMAD]

1. I agree with fallah comments. Shellside DP=9.5 barg, tubeside DP=4.5 barg.
2. Since the shell side design presure 9.5 barg is larger than 4.5 x 1.5 = 6.75 barg then you need to install PSV for tube rupture. Alternatively, you may choose for tubeside DP= 6.5 barg so that tube rupture case can be eliminated
3. However, TRV is requiredd since the tubeside has isolation valves at inlet/outlet.

Edited by S.AHMAD, 11 January 2013 - 11:17 PM.

[kkala]

Post no 7 (by S.AHMAD) adjusts design pressure of the exchanger low pressure side according to 2/3 rule. Low pressure (shell) side design pressure DP= 2/3*9.5=6.33 barg (rounded to 6.5 barg) means previous hydraulic test at 6.33*1.5=9.5 barg (plus adjustments due to design temperature). A tube rupture can "transmit" pressure up to 9.5 barg to the low pressure side, which can be "endured" since hydraulically tested to 9.5 barg (*).
However multiplier of hydraulic test pressure for equipment has changed from 1.5 to 1.3. In this sense low pressure side DP should be adjusted to 1/1.3=0.77 of high pressure side DP to protect exchanger against tube rupture, without PSV for this purpose. In the referred exchanger, low pressure side DP has to get 10/13*9.5=7.30 barg.
We have applied the rule of 10/13 since 2006 for new units and the old rule of 2/3 for new exchangers in old units. This depends on the design pressure "multiplier" applied in the hydraulic test (neglecting temperature adjustments).
Following is also noted.
1. Low pressure liquid piping upstream and downstream exchanger shall have same DP as exchanger's low pressure side.
2. We have specified design pressures according the above rule of 10/13, and also installed TRV (PRV for thermal liquid expansion, in case that the cold stream gets blocked).
3. For new exchangers in new installations in refineries we apply rule 10/13 (or 2/3) "as a rule". Other cases can fit other options (mentioned in this topic).

(*) This is simplified view. Surges occur during tube rupture, http://www.cheresour...-to-pump-rate/'> http://www.cheresources.com/invision/topic/15992-tube-rupture-limited-to-pump-rate/.

[gegio1960]

I share kkala view.

[go-fish]

Thanks to everyone for explaining in detail. That helped a lot.

One more thing to clarify, based on the same analogy in a closed cooling water system where the cooling water passes through various exchangers in the plant and then comes back to the suction of a circulation pump, do I need to design the whole piping system based on pump-off shut off pressure?

In a normal pumping sytem where a liquid is pumped from a vessel through a set of parallel pumps, the DP for discharge piping and DP for suction piping up to the isolation valve are pump shut-off pressure.

Edited by go-fish, 12 January 2013 - 02:34 PM.

[fallah]

We have applied the rule of 10/13 since 2006 for new units and the old rule of 2/3 for new exchangers in old units.

Kkala,

Post No. 6 in below link indicates more updated conditions of above mentioned statement:

Coil Rupture - Relief Devices Forum

[S.AHMAD]

Kkala

TQ for the correction and I totally agree with your comments.

[fallah]

based on the same analogy in a closed cooling water system where the cooling water passes through various exchangers in the plant and then comes back to the suction of a circulation pump, do I need to design the whole piping system based on pump-off shut off pressure?

go-fish,

In such cases the whole cycle piping is to be designed based on pump shut-off pressure considering the static heads of highest and lowest points in the cycle respect to the pump can affect (plus/minus) mentioned basic design pressure.

[Bobby Strain]

To avoid confusion about the "rule". There is not and has never been anything called the "2/3 rule". Simply, the low pressure side test pressure is equal to the high side design pressure to avoid a relief device to protect against tube rupture. Various design codes determine the values.

Bobby