PCHE

[Angsi]

We are looking at using PCHE (heatric) for a clean gas/cooling water service operating at temperature above 50 deg C (but below 100 deg C) in an offshore marine enviroment.

From what I understand, 316L is not accpetable for application above 50 degC in an offshore marine environemnt and due to this cause, application of SS316L, Duplex and Super Duplex have been limited to 50, 100 and 100 deg C respectively.

For the service and temperatures above, vendor has offered the headers constructured of duplex material, but the core is retained as 316L. The core, will be exposed to marine environment.

Can someone please comment further?

 

[metengr]

Please explain what you mean by a marine environment. What is the cooling medium thru the heat exchanger cores?

 

[Angsi]

Marine environment = Tropical salt-laden environment such as experienced on an offshore platform

Service fluid,
Hot side: Compressed clean/dry hydrocarbon gas
Cold side: Cooling water (potable quality) with O2 scavenger injected and controlled chloride content.

 

[metengr]

Based on your input, I see no problems with having the core plates of the Hx fabricated out of 316L ss. These should be protected with a printed circuit type heat exchanger design.

 

[SJones]

The temperature limitation is an empirical one used to minimise the risk of stress corrosion cracking. Stress is the operative word. The core will be a fairly large item so stresses should be lower. Where there could be difficulties is the attachment welds of headers and nozzles etc. Will the equipment be painted?

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[Angsi]

No, it is not going to be painted. The PCHEs are constructed of many "thin" corrugated plates bonded/welded together. Think of it as a plate heat exchanger except that the heat transfer efficiency is much higher which allows the heat transfer area to be reduced, hence its compactness. The plate edges are all welded together and you will not be able to take it apart, unlike the gasketed plate heat exchangers.

The service these exchangers are in is like 2500# class. So I think the plates will indeed be subjected to some stress, hence the concern over CSC.

 

[metengr]

I still don't see how you would have a corrosion problem for these corrugated plates especially if they are bonded by external brazing or welding with the exception of the cooling passages. If I remember, isn't there an external shell that is used to surround the outside surfaces of the corrugated plates to provide for protection? For components like the supply headers or manifolds that are directly exposed to the marine atmosphere, this might be a different matter, which is the reason for upgrading to the Duplex alloy.

 

[SJones]

Just because it's class 2500 doesn't mean that the outer fibres of the core in contact with the atmosphere are highly stressed. If you have your doubts, go to duplex. 316L would require a good maintenance cleaning regime to eliminate salt deposits.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[Angsi]

You may be right, but depends upon which side of the exchanger we are looking at. It is quite like a rectangular block, with six sides. The nozzle headers take up two sides.

In the past the vendor has supplied CRA (duplex/superduplex) shrouds welded on the exchnager to prevent exposure of the SS core to the marine environment. After a while the shrouds developed cracks at the weld area due to frequent "expansion and cooling effect". To overcome this failure, the vendor have now included a breather vent, but I guess anything that breathes out may breath in too, sooner or later.