Standards re: instrument tubing

[NovaNigel]

I'm doing a literature search to find out what standards may provide specific direction on minimum sizes for instrument tubing. The closest I've come is in PIP. Our internal standards lay out these minimums, but I'm having a tough time locating the acknowledged source for these requirements. Does anyone know where I can find a standard that speaks to minimum instrument tubing sizes for impulse, supply, and signal lines?

 

[danw2]

1) Best Practice Guide: Impulse Lines for Differential-Pressure Flowmeters

http://www.flowprogramme.co.uk

2) ISA-77.70-1994 (R2005) Fossil Fuel Power Plant Instrument Piping Installation

The design of instrument sensing and sampling lines in a power plant is covered by ANSI B31.1,
"Power Piping," latest version with addenda.

4.1.1 Process tap connections
All of the material between the root valve(s), including the root valve(s) and process connection,
shall be the same material as the process pipe or vessel or shall be a material compatible with
the process fluid, pressure, temperature, and connection methods. The connection shall be a
minimum of ½ inch (12.7 mm) nominal pipe size for service conditions when pressures are 900
psig (6.21 MPa) or less, and temperatures are 800°F (426.67×C) or less. The minimum size shall
be ¾ inch (19.05 mm) for conditions that exceed either of those limits.

 

[NovaNigel]

Thanks for a very helpful answer, Danw2. This information, coupled with what I found in PIP, gives me the information I need. (This is the first time I've posted a thread. The speed of response, and informative reply is terrific!)

 

[roydm]

Holly cow, who would dream of running impulse lines 90 meters, 16 for that matter.
I think the line size is determined more by mechanical strenght than anything. 3/8 or 1/2" is self supporting to a degree wheras 1/4" needs lots od support.
My 2c worth
Roy

 

[JLSeagull]

Often, company standards dictate the tubing selection instead of industry standards. Many companies are responding with new requirements due to poor experience with low quality tubing. Stay tuned for lots more QA testing required for tubing.

Length
In addition to impulse lines, instrument tubing includes sample transport tubing. Although 90 m is a long run it is not uncommon.

Diameter
The most common impulse lines include 1/2-inch OD; and 3/8-inch OD tubing is sometimes used for impulse lines in natural gas plants. Properties such as high pressure or high viscosity affect the tubing selection.

Analyzers often use 1/16-inch OD tubing (within the oven of a GC for example).

Material
Copper was commonly for air supplies and even steam tracing fourty years ago. Copper is rarely permitted by most clients in the hydrocarbon business today.

For process, the piping material is regarded as the "minimum" material. In my experience, type 316 stainless steel has been the most commonly used tubing for instrumentation in a carbon steel line specification. However recent trends to higher alloys are becoming common. This appears to be in response to low grade tubing in the market place. I don't know the origin of the problematic tubing. Some consider type 317 even for air due to poor performance of type 316 in offshore environment. At least one major operating company is actually using alloy 625 or 825 for impulse lines in lieu of type 316 stainless steel. These nickel alloys are very expensive.

 

[roydm]

I have seen long runs years ago when all the instrumentation was pneumatic but why would you do that these days? Given that tubing is expensive, cable is cheap and the signal ends up in the DSCS or Safety Shutdown System it makes no sense, long tubing runs the risk of potential errors. I always try to keep the runs less than a length of tubing. The longest ones would be for DP across a tower packing and they run straight up.
Roy