Space is typically at a premium where offshore installations are concerned. The requirement to accommodate large rotating machinery and its associated piping take priority over the needs of field instruments, and, in particular, over in-line instruments. In addition, fabrication contracts are increasingly run on a fast-track basis and sometimes without adequate on-site instrument design support. The combined result is often non-ideal positioning of orifice-plate assemblies and their associated impulse lines. Such non-ideal cases should be avoided if at all possible. However, if left with an installation of this sort, it is unlikely project management will be receptive to reconfiguring the process piping, particularly if large lines are involved. The arrangements provided here may be of use in achieving an adequately functioning installation under these circumstances.
The following guidelines assume horizontal process piping and remotely mounted transmitters. For vertical process lines, the simple rule is to install gas service orifice-plate assemblies such that flow is downward. This allows any condensate to flow away from the impulse line tapping points. Were the flow to be upward, then any liquid droplets would tend to move downward against the flow and collect around the edge of the orifice bore. This could introduce noise on the measurement. For orifice-plate assemblies in liquid service, the flow should be upward such that any entrained gas is encouraged to vent away from the tapping points. Were the liquid flow to be downward, then entrained gas would tend to rise and once again could create measurement noise. Care must be taken for liquid service in vertical lines to compensate for the difference in height of the upstream and downstream taps.
Close-coupled differential-pressure flow transmitters are desirable because they provide quick response times. However, high process temperatures or excessive pipe vibration can preclude close coupling. A need to gain regular access to the transmitter can also be a factor, where the orifice-plate assembly has been installed significantly above or below accessible deck level. Should a close-coupled transmitter be used in such a situation then a remote indicator may well be required at grade level.
Ideal Impulse-Line Installation
Gas Flow Measurement: Figure 1 depicts the ideal impulse-line installation for gas flow measurement where no access limitations exist. Orifice-plate tapping points should be positioned either at the top of the line or between the top of the line and the 45-degree position. Any condensate in the process line will collect in the lower quadrant of the pipe. Should entrained liquids enter the impulse lines, then on condensing the liquids will tend to drain back toward the process piping.
Liquid Flow Measurement: Figure 2 depicts the ideal impulse-line installation for liquid flow measurement where no access limitations exist. Orifice-plate tapping points should be positioned at the 45-degree position in the lower quadrant. Any gas in the process line will collect in the upper quadrant of the pipe. Should entrained gas enter the impulse lines, then it will tend to vent back towards the process piping.
The tapping points should not be taken from the bottom of the pipe, as this position could expose the impulse lines to accumulated solids and result in blockage.
Impulse-Line Installation Under Non-Ideal Conditions
Gas Flow Measurement: Orifice-plate assembly installations are sometimes packed tightly beneath solid decks, such that gas tapping points have to be taken from the lower quadrant of the line. Under these conditions, and unless the gas is completely dry, it is necessary to install drain points on both transmitter impulse lines to prevent buildup of condensate (Figure 3).
Drain lines may require piping to a closed system, depending on the nature of the fluid being measured and the hazard that it could represent. Each drain line should include a length of tubing below the level of the transmitter, terminating in a quarter-turn isolation valve. Condensate can then build up in the drain leg without adversely impacting measurement and can be periodically removed. Ideally, two valves should be installed in series, with a length of tube separating each. The first (upper) valve is normally open and the second (lower) valve is normally closed. On draining, the first valve is closed and the second valve opened. In this way the condensate can be transferred to the drain without depressurizing the transmitter leg. Without this second valve, it may be necessary to temporarily isolate the transmitter from the process signal via the five-valve manifold.
An alternative arrangement is to install a drain pot in place of the length of drain line tubing. This allows a larger volume for condensate to collect in and requires less frequent drainage operations (Figure 4).
This arrangement should be used with caution, however, as the extra compressible volume provided by the drain pot could slow the transmitter response time. API RP 551 recommends a maximum impulse line length of 20ft (6m). The drain pot volume can equate to a significant length of impulse line. A two-inch nominal diameter drain pot made from schedule 80 SS pipe represents, per foot, the rough equivalent volume of 20ft of ½" OD, 0.049-inch wall SS tubing.
Liquid Flow Measurement: Orifice-plate assembly installations are also sometimes placed close above grade level on solid decks, such that liquid tapping points have to be taken from the upper quadrant of the line. Under these conditions, it is necessary to install vent points on both transmitter impulse lines to prevent buildup of gas (Figure 5).
Vent pots can also be installed in the impulse lines, similar to the alternative arrangement shown in Figure 4.
Impulse-Line Diameter
API RP 551 recommends the use of ½" OD tubing minimum for differential-pressure instrument impulse lines. Process Industry Practices PCCIP001 recommends the use of 3/8" OD tubing minimum impulse lines, although it is understood this guidance is currently being revised to ½". The smaller diameter has the advantage of minimizing the amount of fluid that would pass to atmosphere due to a line rupture. It also has the advantage of representing less volume, thus reducing response times on compressible service. However, the larger diameter line would be less likely to plug on dirty service. Also, a ½" tube would be less likely to shear than a 3/8" line of the same wall thickness, due to the former’s larger cross-sectional area.
Other Considerations
Other important installation considerations include minimum piping straight-length runs, the use of dual isolation impulse line valves for higher pressure service, the application of purpose-built valve manifold assemblies in place of separate isolation valves, and the use of liquid seals.
(EDITOR’S NOTE: Steam service is not frequently found in offshore applications, apart from cleaning, and so is not covered here. Heat tracing, when required, is usually accomplished via electrical means.)
Andrew Bartlett, P.E. is a technical professional leader at KBR Inc. in Houston, Texas. Mr. Bartlett is a registered Professional Engineer in Texas and is also registered as a Chartered Engineer in the United Kingdom. He holds both a master’s degree and bachelor’s degree in Electrical/Electronic Engineering, as well as an MBA. Mr. Bartlett has worked in the field of oil and gas developments for over 25 years in design, construction, commissioning, and operations support. Mr. Bartlett can be reached at [email protected].
www.kbr.com
References
1. API RP 551 (1993) – Process Measurement Instrumentation BS 6739 (1986) – Code of practice for instrumentation in process control systems: installation design and practice.
2. PIP PCCIP001 (2002) – Instrument Piping and Tubing Systems Criteria.
