Success factors for Orifice Meter Installations
Orifice Meter Advantages
Jerry points out, 'The design of orifice meters is simple, yet they are accurate and versatile. They can be used to measure most fluids - gas, liquid, steam and even two-phase fluids with a small amount of second phase. Orifice accuracy can be maintained in liquid streams with as much as 5% (by volume) gas bubbles and in gas streams with up to about 2% (by weight) of solids or liquids.'
He continues, 'Another factor in orifice popularity is the breadth of options available for sizes, pressure ratings, trim, flow rates, and other characteristics. For example, the Daniel Senior(R) Orifice Fitting - the premier dual-chamber fitting with a worldwide reputation for high accuracy, reliability, ease of use, maintainability and safety - is available in 2 inches through 48 inches sizes. Pressure ratings go to 2500# ANSI and 10.000# WOG, the highest in the industry.
Field repairable, the meter meets API '14.3' and ISO 5167 without exceptions or penalties. Its time-proven, lubricated slide valve to isolate upper and lower chambers is available in a 'soft- seat' version for bubble-tight sealing.'
Three trims are available, and Jerry suggests that if there is any doubt about which trim to specify or if special materials are needed, it is always safer to contract a Daniel specialist for advice. Seals are available made from nitrile, Teflon, and metal-to-metal. 'DVS' seals are vulcanized to the orifice plate for best operation in larger sizes. Hydrostatic test to 1.5 times the rated pressure and both high- and low-pressure plate seal test are standard.
'Three important success factors for any orifice meter installation,' says Jerry, 'are complete specifications and/or application details, correct field installation, and proper maintenance.
Specifications should include nominal bore diameter based on maximum and minimum gas volume to be measured for the immediate future along with the third and tenth year of use. If gas quality is other than 'normal pipe line quality,' it should be noted when ordering. An order should also include notation about any standards or codes involved, according to Jerry.
Daniel Orifice plates are inspected with a Coordinate Measuring Machine to insure compliance with customer, Daniel, and standards' requirements. Unless otherwise specified, plates will be beveled to the smaller of 1/50 of the line ID or 1/8 of the orifice bore. Special designs include: counterbores to limit plate-edge thickness (no bevel), segmental for use where solids are entrained in gas or liquid flow, eccentric bore to help pass solids or slurries, and quarter round for viscous media with Reynolds Numbers below 100,000.
Field Installation and station design directly impact meter performance. How many meter runs will be required? What valving, if any, will be needed? What noise and vibration problems need to be anticipated?
Jerry suggests, 'A useful rule-of-thumb for proper multiple meter runs is to minimize velocities in the header by making its area 1-1/2 to 2 times the combined areas of all meter tubes connected to it. If proper distribution is not achieved, it is possible that with high flow rates one tube will take excessive flow - with resulting high differential - before the other tubes reach their maximum flow capacity. Station throughput will be unnecessarily limited.'
A major reason why Senior fittings are so popular is that no extra valving or piping is needed to inspect or change a plate without interrupting line flow. For single-chamber fittings, one of three control valve systems is commonly used: direct-acting or self-actuated regulators, pilot-operated regulators or control valves, controller-operated control valves. Most operators choose tile simplest configuration that will meet station needs.
The importance of how much noise will be caused by high flow velocities varies with station location while vibration will be a problem anywhere. Dynamic fluid noise and vibration will generally be acceptable when flow velocity underground is limited to 200 fps and in above-ground piping to 100 fps. More sophisticated analysis of noise and vibration can be achieved considering mass-velocity values instead of simply velocity alone.
Proper maintenance is essential to insure accurate readings and long service life. Explains Jerry, 'One of the best ways to reduce 'lost and unaccounted for' gas (or corresponding liquid losses) is to maintain orifice fittings and plates properly. The periodicity of plate inspections is best determined by frequent inspection until enough information is obtained to determine the optimum timing of routine inspections. Should flowing media, temperatures or pressures change significantly, inspect more frequently until a new routine can be defined.
Daniel provides suggested routine steps to help keep Senior fittings operating properly, including fitting operation at least monthly and checking drain-plug holes to prevent accuracy- destroying debris buildup.'
Jerry cautions, 'If sediment is likely to accumulate in the fitting, install a blowdown valve in place of the pipe fitting. Make sure the plate carrier is raised before blowdown or cleaning through blowdown valves. A pigging ring is available to help prevent dirt from being forced into the plate carrier cavity in the fitting during pigging operations.'
And some additional cautions. Don't install a blank orifice plate to block off the line; it can't hold high pressures and may be blown down the line or 'dished' so badly that it won't crank out. Use only factory certified Daniel parts; you cannot see metallurgical quality, stress concentrations or dimension details. What looks alike may actually be importantly different.
By following the simple steps outlined, you can greatly increase your chances for a successful installation.