A refined manifold
9 Mar 2005
For instrumentation on the high-pressure ethylene pipeline that connects Grangemouth with BP's chemicals manufacturing site at Hull, the company wanted to use the 'threadless connector' variant of the Rosemount 3051 pressure transmitter, which offers a flat connection face with an O-ring style seal.
Conventionally, this might require the use of a kidney flange component to convert the face to a compression tube fitting - a connection solution with two potential leak paths. However, Parker Instrumentation's regional distributor Hydrasun thought an even better answer was possible and after discussing it with Parker's R&D lab in Barnstaple, suggested to BP Grangemouth that they could produce an application-specific manifold with a directly compatible interface.
Manufactured from a single forging, this manifold reduces the leak paths to the absolute minimum of one, and connects by simply aligning the faces and tightening four bolts. Incorporated into the body of the manifold are three valves providing an industry-standard 'double block and bleed' facility to aid installation and maintenance.
'This new manifold provides a high integrity double block and bleed instrument connection facility with a height of just 14cm, and has been designed in such a way that we can quickly modify the instrument connection face to suit other threadless connection options offered by process instrument vendors,' says Steve Jones of Parker Instrumentation.
Editor's note: Sealants are required for taper threaded connections because of fundamental design principles related to the shape of threads, which mean that joints leave tiny gaps that may leak under pressure. It is possible to get 'dry thread' types which avoid this situation, but they are basically an option for one-time use and not suitable for this application.
This is compounded by the fact that the stainless steel materials preferred by the oil and gas industry are prone to galling - or cold welding - if no lubricant is used on threaded connections. This can lead to the scrapping of expensive components.
The remedy is liquid sealant or PTFE tape. However, it's easy for these materials to then become contaminants in the instrumentation system, largely because of imprecision with which they are applied. Contaminant particles can disrupt the instrument's operation, a problem which might have serious consequences; consider the case of an instrument used for custody transfer or fiscal metering for example. And, if that contamination does not happen during initial assembly, it's even more likely to happen when the instrument is in operation, and/or when it is periodically disassembled for routine maintenance - because the old tape or sealant has become brittle/flaky/dry.
There are guidelines for sealant application of course, but sadly, no industry standard. The trouble is that tape thicknesses vary, and liquid sealants do not come in pre-loaded forms which deliver precise quantities. It is very easy to over- or under-apply the materials.
