HX Design Pressure and Design Temperature Decision for a Pumped System (Contd).

[Pavan Kumar]

Hi All,

In reference to the thread below, I estimated the shell and tube side design pressures as follows:

https://www.eng-tips.com/viewthread.cfm?qid=497097

1. Shell Side

Pump shut-off pressure = 46.5 psig
Design pressure = 1.25*46.5 = 58.125 psig at coincident temperature of 57 Deg C
Design Temperature = 57 Deg C

2.Tube Side

Pump Shut-off pressure = 73.5 psig
Design Pressure = 1.25*73.5 = 92 psig at coincident temperature of 32 Deg C.
Design Temperature = 32 Deg C.

In summary :

Shell Side DP = 60 psig at 57 Deg C
Tube Side DP = 92 psig at 32 Deg C.

My question is should I add a safety factor for my design temperature or not. The temperatures mentioned are the maximum possible temperatures in the system.

Thanks and Regards,
Pavan Kumar

https://www.eng-tips.com/viewthread.cfm?qid=497097

 

[LittleInch]

Well you added a factor of 1.25 for the design pressure for some reason which you didn't explain in the previous post, so why not temperature.

Process engineers always seem to have a desire to add margins al over the place as it has no consequence for them, only the mechanical and piping engineers who have to take their numbers and make them work.

It all depends on how certain you are about the pressure and temperature maximum figures.

Also another unanswered comment is how on earth can you have a different design temp for the shell and tube.

At this stage for the vessel design, you cannot (IMHO) eliminate the potential that one flow or the other won't keep going whilst the other is no flow. Or even that the shell side is full of water so even if it stops at the same time as the tubes, the temperature of both will equalise.

So set both at 60C would be my recommendation and think about the pressures.

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

 

[Pavan Kumar]

Hi LittleInch,

Well you added a factor of 1.25 for the design pressure for some reason which you didn't explain in the previous post, so why not temperature.

The safety factor of 1.25 is arbitrarily decided. I used this safety factor to accommodate for increased flow later on - say if the pump is changed in the future. I wanted to know what is the rule of thumb for safety factor over maximum possible pressure and maximum temperature.

Also another unanswered comment is how on earth can you have a different design temp for the shell and tube.

The shell side service is Tepid water which is being cooled from 57 to 47 Deg C while the tube side is cooling water which is being heated from 25 to 32 Deg C. That's the reason for two different DTs on shell and tube side.

At this stage for the vessel design, you cannot (IMHO) eliminate the potential that one flow or the other won't keep going whilst the other is no flow. Or even that the shell side is full of water so even if it stops at the same time as the tubes, the temperature of both will equalise.

Yes in the operation of this HX it is possible that the shell side flow is stopped while the cooling water flow is ON. To be on the safe side you recommend both the shell and tube sides to have the same design temperature correct. In that case I will use 90 Deg C was the design temperature for both than 60 Deg C. The plate thickness will be bit higher due to this higher design temperature.

Please let me know what you think.

Thanks and Regards,
Pavan Kumar

 

[MJCronin]

This guideline has an alternative recommendation on design temperatures and pressures: (See page 7)

:Design temperature and pressure. Calculate the design temperature and pressure on both the shellside and the tubeside by adding an appropriate safety margin to the maximum values expected in service. Consider the following guidelines:

1. To arrive at the design temperature, add a margin of 30°C (50°F) to the maximum allowable operating temperature of the exchanger.

2. Similarly, the design pressure can be calculated by adding an appropriate margin to the maximum allowable operating pressure.

3. If the process hazard analysis has identified tube rupture as a hazard, to avoid the need to design a pressure relief valve for the tube rupture case, the shellside design pressure must be at least 77% of the tubeside design pressure (the “10/13” rule). For instance, if
the tubeside design pressure is 500psig, the minimum shellside design pressure should
be 500 × 10/13 = 385psig. The logic of this is that ASME codes require the shell to be
hydraulically tested at 1.3 times its design pressure, so tube rupture — which is generally
considered an unlikely event — would not pressurize the shell beyond its test pressure.

4. When deciding the design temperature, consider routine operations such as steaming of the heat exchanger during maintenance.

http://stoppingclimatechange.com/Shell-and-Tube Heat Exchangers.pdf

MJCronin
Sr. Process Engineer

 

[danschwind]

Well, you pretty much ignored my post in your last topic which, IMO, covers all your questions.

Daniel
Rio de Janeiro - Brazil

 

[georgeverghese]

Its good practice to state both lower and upper design temperatures.

 

[Pavan Kumar]

Thank You All for the guidance provided. I will use these inputs to set the design pressure and temperature for my HX.

I apologize to danschwind. I really did not ignore your guidance. Your suggestions went out of my mind in the mix of various of activities. I will be more attentive.

Thanks and Regards,
Pavan Kumar

 

[LittleInch]

The issue of margin over maximum operating is one which goes around a lot.

As you can see from the paper from NJC, process engineers like margins of a decent size.

However they do cost more.

For pressure, so long as you have safety devices and control, 10% is probably the minimum you can use to allow for alarm and trip settings plus a margin for the relief valve or device you need for any PV.

Temperature is a bit more dependant on the process that is driving the temperature. If there is potential for higher temp in an upset condition then use that. If not then add 10C or 10%. I don't like adding a fixed amount as large as 30C, especially on low temperatures. Also be alive to where this margin pushes you into a higher pressure class or increased thickness.

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

 

[Pavan Kumar]

Hi LittleInch,

I will keep points in mind before selecting the Design Pressure and Temperature. Thank you very much for your inputs. I will keep you posted on the final values I decide and get your inputs on those by the end of this week.

Thanks and Regards,
Pavan Kumar