6 replies to this topic

[Santaex]

Hi,

I would like to conduct a heat exchange with 2 streams. One of them is condensing stream at 1 bar and 153.1C located at the condenser of a distillation tower. The other is a process stream which is going to be heated from 43.7C to 140C at 30 bar.

 

I understand there is a 2/3 rule which only allows my condensing stream to pair up with another stream which at most 1.5 times its pressure. Is there anyway to work around the 2/3 rule?

Perhaps setting up a heat transfer oil loop with built in pump and valves to control the pressure? Or installing any safety devices?

[fallah]

Sanataex,

 

Below link (especially post No. 6) can help you regarding new looking of API 521 about design pressures (previous 2/3 or 10/13 rules) of low/high pressure sides of the shell and tube heat exchangers in tube rupture case.

 

 

http://www.cheresour...7-coil-rupture/

Edited by fallah, 02 April 2013 - 01:24 PM.

[shan]

Just install a rupture disk as a prompt pressure relief device in case of heat exchanger tube rupture.

[paulhorth]

Santaex,

 

There is no rule about which streams can be "paired up" in a heat exchanger. You can exchange heat between streams at any pressure you like.

However, you must provide overpressure protection for the low pressure side in case of a burst tube, which will allow pressure through from the high pressure side. This overpressure protection is usually a rupture disc because it is faster acting than a PSV.

 

Now, rupture discs are inconvenient things, I consider them to be devices of the devil. If you can avoid using them you will have a better design. The rule that you are thinking of is a provision in the design codes ASME VIII and ISO 23251 which allows overpressure protection to be avoided if the HYDROTEST pressure of the LP side is greater than or equal to the MAX OPERATING pressure of the HP side.  This condition equates (approximtely but not in every case) to the MAX OP pressure of the HP side being 13/10 x the design pressure of the LP side. Some companies use the DESIGN pressure not he MAX OP pressure.

 

To take advantage of this provision, you need to raise the design pressure of the LP side (the operating pressure can stay the same), this is sometimes worth doing to eliminate the rupture discs, but in the case you give it is probably not worthwhile because the HP side pressure is much higher than the LP side.

 

Note that the provision applies only to tube rupture overpressure, you still need to consider other sources of overpressure such as fire or thermal expansion.

 

Paul

[Raj Mehta]

Hello Mr. Paul,

 

Can you please explain your above post with some hypothetical numbers for pressure or with those pressures mentioned in the first post ?

Edited by Raj Mehta, 05 September 2013 - 02:00 AM.

[gegio1960]

we have to go by assumptions, since the design presures of the two systems are not given.

so, let's assume LP side has 3.5 barg design pressure while HP side has 33 barg Pdes.

you've to increase the Pdes of the LP side of the Heat Exchanger to 33x10/13=25.4 barg to apply the rule.

the rule is often applied to coolers with cooling water in the tubes (LP side).

[gegio1960]

the content of the following topic is very important when working on this systems

http://www.cheresour...ers/#entry74585