The use of unions

[lgoochy]

Can anyone out there help.

I am in the process of designing some loop pipework to support a mechanical seal. The user has a preference for flanged pipework, however the pipework must terminate at a screw thread to enable it to be screwed into the seal.
I thought about using slip flanges to connect to the rest of the pipework, however we are on a tight budget and are thinking about using a union so that we can line up the flanges correctly on installation.

The fluid going through the pipework is oil at around 70 Deg C and we are sealing a hazardous hydocarbon which in theory should not come into contact with the pipework we are supplying.

The units are going into a refinery and they have specified API 682. Has anyone used unions on this type of application before.

Any replies would be greatly appreciated

 

[Montemayor]

Plumbers use unions; engineers use flanges.

 

[zdas04]

Art,
Have you ever been on an onshore natural-gas wellsite? I don't qualify as a plumber, but I've spec'ed thousands of hammer unions outside of plant fences.

David

 

[GSTeng]

I think you struck a cord with David there.

I think what David means is that unions are used all over the oil and gas industry for the exact function that you are describing. Unions are use all over enigne oil systems that are around Hydrocarbon environments and are at higher temperatures than you are describing.

But if your customer is completely against using threaded fittings then you will have to look at other options. Ask the customer what he would prefer at that connection.

 

[dcasto]

I'd rather have a 1" or smaller union over a 1" slip flange for integrety anytime.

 

[nosabe]

I work in an oil refinery(15 yrs)I've seen unions installed in piping systems with temperatures a lot higher than 70 deg.C without any problems

 

[dtn6770]

Unions have their place; a petroleum refinery I spent 14-yrs at allowed them in generic hydrocarbon service in piping below NPS 2 in 300 class flange (carbon steel) systems.

 

[nosabe]

plumbers use unions; engineers use flanges?...mmmmm OK

 

[JohnBreen]

I have seen stainless steel unions used (with metalic hoses) in nuclear power plants. Component Cooling Water to the control rod drive mechanisms. I assure you it was a decision made by ENGINEERS.

 

[JLSeagull]

Avoid unions where practical. Other unpopular items include bushings and bell reducers (we like swage nipples instead).

API RP 550 was an old standard for installing instruments that was withdrawn. API RP 550 showed hard pipe and unions to install the mercury manometer dp transmitters of the 1950's.

The current API RP 551 standard shows "orifice-unions" for purge connections on some level measurement applications. Unions are often used in non-pressure containing thermowell head connections. I am not aware of other applications for unions.

ASME B31.3 lists unions in 314.2.2.

 

[lgoochy]

guys

Thanks for all your posts. I think what I really need to do is put it to the customer and find out what they are willing to accept.

Many thanks once again for the time and effort taken to reply