API MPMS 4.9.1 pdf download.Manual of Petroleum Measurement Standards Chapter 4—Proving Systems
a. The manual-return unidirectional prover, sometimes referred to as the measured distance, is an elementary form of an in-line prover that uses a section of pipeline as the prover section. Prover detector switches that define the calibrated volume of the prover section are placed at selected points along the pipeline. A displacer-launching device is placed upstream from the prover section, and receiving facilities are installed at some point downstream from the prover section. Conventional launching and receiving scraper traps are usually used for this purpose. To make a proving run, a displacer (a sphere or specially designed pis- ton) is launched and allowed to displace the reference volume before being received downstream and manually transported back to the launching site. This type of prover is no longer in common use. b. The circulating-return unidirectional prover, often referred to as the endless loop, has evolved from the prover described above. In the endless loop, the piping is arranged so that the downstream end of the loop crosses over and above the upstream end of the looped section. The interchange is the means by which the displacer is transferred from the downstream to the upstream end of the loop without being removed from the prover. The displacer detectors are located inside the looped portion at a suitable dis- tance from the inter-change. Continuous or endless prover loops may be automated or manually operated.
5.3 DISPLACEMENT TYPE METER PROVERS WITH CAPTIVE DISPLACERS
• Unidirectional Piston Provers with Optical (external) Detectors A prover having a captive displacer has an attached shaft or rod, which moves with the displacer. The displacement of the shaft is constant except that it moves into and out of the calibrated section during a calibration run. Thus a prover with a captive displacer and a shaft attached to one end will have an upstream and a downstream volume. If a shaft is attached to both sides of the dis- placer and they have equal area displacement, the upstream and downstream volumes are equal. There is often one or two other detector and/or guide rods attached to the captive displacer. Since these type provers have externally mounted optical detectors, the thermal effect on the steel may not be the same for the area aspect as for the linear aspect. For example, if both the prover chamber and the mounting that defines the linear distance between the detector(s) were the same, the thermal effects would be the same. But in many cases, a prover chamber might be made of a type of stainless steel while the detector(s) mounting which defines the linear distance between the detector(s) might be made of a special alloy having a different thermal coefficient of expansion. In terms of a prover calibration, the main effect is that it is necessary to obtain both a prover barrel temperature and a detector temperature.
6.1 PROVER DETECTOR SWITCHES
A detector switch is a high precision device mounted on a prover, which is used to detect the passage of a displacer. The calibrated volume of a prover is the amount of fluid that is displaced between two detector switch positions. Additional detector switches may be used if more than one calibrated volume is required on the same prover, or they can also be used to signal the entrance of a displacer into the sphere resting chamber. Several types of detector switches are described below. 6.1.1 Mechanically Actuated Detector Switches The mechanical type of detector switch is used primarily with an elastomeric sphere displacer, but there are applications where they are used with piston displacers, and are operated when the displacer contacts a rod or ball protruding into the prover pipe. At the point of operation a switch is closed or opened by means of a mechanically or magnetically driven contact. Mechanically actuated detectors may or may not be pressure balanced. Pressure balanced detectors have ports or passages that allow pressure to be equally distributed on the switch rod, thereby offsetting the effect of pressure on the activation of the detector.