1、 - - - 4 CEN EN*IS0*7278- 2 95 U 3404589 0131825 247 BRITISH STANDARD Liquid hydrocarbons - Dynamic measurement - Proving systems for volumetric meters Part 2. Pipe provers The European Standar EN Is0 72782 : 1995 has the status of a British Standard BS EN IS0 7278-2 : 1996 Incorpomting Amenrtment N
2、o. 1 BS 6866 Part 2 ,1990 renumbered NO COPYING WlTHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAW 8899 - CEN EN*IS0*7278- 2 95 W 3404589 OL3L82b LB3 June 1996 indid by a sideline in the margin s-.K*-w - 6 BS EN IS0 7278-2 : 1996 Me 2, June 1996 Committees responsible for this British Stand
3、ard The preparation of th the standard reference conditions referred to in 1.3 are identical to hose given in Bs 5579. Compliance with a British Standard does not of itself confer immunity from legal obligations. . . . ii Q BSI 1996 CEN EN*IS0*?278- EUROPEAN srmm NORME EUR0PEE”E EuROPmCHE NORM 2 95
4、rn 3404.589 OL3L830 604 rn EN IS0 727 proof: The determination of the meter factor 3.6 lowest values within a batch of results. range: The difference between the highest and the 4 Description of systems 4.1 General 4.1.1 There are several types of pipe prover, all of which are relatively simple and
5、commercially available. All types operate on a common principle, namely the precisely measured displacement of a volume of liquid in a calibrated section of pipe between two signalling detectors, by means of a displacer (a slightly oversized sphere or piston) being driven along the pipe by the liqui
6、d stream being metered. While the displacer is travelling between the two detectors, the output of the meter is recorded automatically. Pipe provers may be operated auto- matically or manually. 4.1.2 A meter being proved on a continuous-flow basis shall, at the time of proof, be connected to a count
7、er which can be started or stopped instantly by the signalling detectors. The counter is usually of the electronic-pulse-counting type. The counter is started and stopped by the displacing device ac- tuating the detector at each extremity of the calibrated section. 4.1.3 There are two main types of
8、pipe prover: unidirectional and bidirectional. The unidirectional prover allows the displacer to travel in only one direction through the proving section, and has a transfer arrangement for returning the displacer to its starting position. The bidirectional type allows the displacer to move first in
9、 one direction, then in the other. It therefore incor- porates a means of reversing the flow through the pipe prover. (See figures 1, 2 and 3.) 4.1.4 Both unidirectional and bidirectional provers shall be constructed so that the full flow through the meter being proved passes through the prover. 4.2
10、 Unidirectional provers 4.2.1 Unidirectional provers may be subdivided into two categories depending on the manner in which the displacer is handled, namely the manual-return in-line type sometimes referred to as a “measured distance“ type, and the autornatic- return or circulating type, often calle
11、d the “endless loop“ type. a) The manual-return unidirectional prover is an elemen- tary form of in-line prover which uses a section of pipeline as the prover section. The entire metered stream may flow continuously through the prover even when the prover is not being used for proving. Detectors are
12、 placed at selected points which define the calibrated volume of the prover sec- tion. A displacer launching device is upstream of the prover section, and receiving facilities are installed at some point downstream of the prover section. Usually, conventional launching and receiving scraper traps ar
13、e used for this pur- pose. To make a proving run, a displacer (a sphere or specially designed piston) is launched, allowed to traverse the calibrated section, received downstream and then manually transported back to the launching site. b) The automatic-return unidirectional (endless loop) prover ha
14、s evolved from the prover described in 4.2.1 a) and is shown in figure 1. In this endless loop, the piping is arranged so that the downstream end of the looped section crosses over and above the upstream end of the loop. The interchange is the means whereby the displacer is transfer- red from the do
15、wnstream end to the upstream end of the loop without removing it from the prover. The displacer detectors are located at a suitable distance from the inter- change inside the looped portion. Such endless prover loops may be manually operated or they may be automated so that the entire sequence for p
16、roving a meter can be ac- tuated by a single action. The metered stream may be per- mitted to run through the prover when the prover is not being used for proving, and the prover need not be isolated from the carrier line unless desired. This permits the move- ment of several different types of liqu
17、id in succession through the prover, and affords a self-flushing action which minimizes intermixing between them, as well as providing temperature stabilization. 4.2.2 A meter proof run in a unidirectional prover consists of a single one-way run, therefore the base volume of a unidirec- tional prove
18、r is the volume of liquid, corrected to standard temperature and pressure conditions, displaced between the detectors during a single trip of the displacer. 4.3 Bidirectional provers The bidirectional prover has a length of pipe in which the displacer travels back and forth, actuating a detector at
19、each end of the calibrated section and stopping at the endof each run when it enters a region where the flow can bypass it or when the action of a valve diverts the flow. Suitable sup- plementary pipework and a reversing valve, or valve assembly, either manually or automatically operated, make possi
20、ble the reversal of the flow through the prover. The main body of the prover is often a straight piece of pipe (see figure 21, but it may be contoured or folded (see figure 3) so as to fit in a limited space or to make it more readily mobile. Normally, a sphere is used as the displacer in the folded
21、 or contoured type and a piston is used in the straight-pipe type. A meter proof run usually consists of a “round trip“ of the displacer, and the displaced volume in this type of prover is expressed as the sum of the displaced volumes in two consecutive one-way trips in opposite directions. CEN ENxI
22、S017278- 2 95 3404589 0131834 25T IS0 7278-2 : 19 this can usually be achieved by inflating the sphere to a diameter which is at least 2 % greater than the inside diameter of the prover pipe. In general, the larger the sphere, the greater this percentage should be. Too little expansion of the sphere
23、 can lead to leakage past the sphere and consequent measure- ment error. Too great an expansion of the sphere may not im- prove sealing ability and will generally cause the sphere to wear 3 CEN EN*ISO*7278- 2 95 D 3404589 0131835 L9b D IS0 7278-2 : 1- (E) more rapidly and to move erratically. Care s
24、hall be exercised to ensure that no gas remains inside the sphere. The elastomer shall be as impervious as possible to the operating liquids and retain its mechanical properties (especially its elasticity) under operating conditions. The liquid employed to fill the sphere shall have a freezing point
25、 below any anticipated temperatures. Water or water-glycol mixtures are commonly employed. 6.5.2 A second type of displacing device is the cyindrical piston with suitable seals. This is often used with straight pipe provers that have been internally honed to ensure adequate sealing. 6.5.3 Other disp
26、lacers are acceptable if they give a perform- ance equal to the two types mentioned in 6.5.1 and 6.5.2. 6.6 Valves 6.6.1 All valves used in pipe prover systems which can con- tribute to a bypass of liquid around the prover, the displacer or the meter, or which can cause leakage between prover and me
27、ter, shall be bubble-tight on low differential pressure tests. A means of checking valve seal leakage during the proving run shall be provided for such valves. If a sphere or spheres are used to provide this sealing mechanism in lieu of a valve, they shall be provided with some means of testing for
28、leakage. 6.6.2 The entire operation of the flow reversing valve or valves in a bidirectional prover, or of the interchange valve in a return type unidirectional prover, shall be completed before the displacer actuates the first detector. This is to ensure that dur- ing the trip of the displacer thro
29、ugh the calibrated section no li- quid is allowed to bypass the prover. The necessary distance between the initial position of the displacer and the first detec- tor, commonly called the pre-run, is dependent on valve opera- tion time and the velocity of the displacer. Any method can be used to shor
30、ten this pre-run, whether by faster operation of the valve or by delaying the launching of the displacer. However, caution shall be exercised in the design so that hydraulic shock or additional undesired pressure drop is not introduced. If more than one flow directing valve is used, all valves shall
31、 be linked by some means to ensure that shock cannot be caused by in- correct sequence of operation. 6.7 Calibration connections Connections shall be provided on the prover to allow for water draw or master meter calibration at a later date (see figures 1,2 and 3). 6.8 Detectors Detection devices an
32、d switches shall indicate the position of the displacer accurately, and in a bidirectional prover they shall operate equally well in both directions. Various types of detec- tor are in use, the most common of which is the mechanically actuated electrical switch. Other types, including the electronic
33、 proximity, the induction pickup or the ultrasonic type, may be used, provided the required repeatability criteria are met. The precision with which the detector in a prover can detect the posi- tion of the displacer (which is one of the governing factors in determining the length of the prover sect
34、ion) shall be ascertained as accurately as possible (see annex A). The diameter of any opening in the wall of the calibrated section of the pipe, including the holes accommodating the detectors, shall be appreciably less than the width of the sealing zone of the displacer. 6.9 Meter pulse generator
35、An externally fitted pulse generator shall generate electrical pulses of satisfactory characteristics for the type of proving counter employed. The device shall generate a sufficient number of pulses per unit volume to provide the required resolution. The pulse emitter shall be designed to eliminate
36、 the generation of spurious pulses due to mechanical vibrations or other influences. 6.10 Electronic pulse counter An electronic pulse counter is usually used in meter proving because of the ease and accuracy wi?h which it can count high- frequency pulses and because of its ability to transmit its c
37、ount to remote locations. The pulse-counting devices are equipped with a start-stop electronic switching circuit actuated by the provers detectors. Proving systems can also be equipped with a pulse interpolation system as defined in IS0 727-3. 6.11 Equipment for automatic-return unidirectional prove
38、rs 6.11.1 Equipment necessary for the proper operation of the automatic-return or endless-loop unidirectional prover is cen- tred around the sphere interchange unit. It is within this unit that the sphere is diverted from the flowing stream at the downstream end of the prover, passes through the int
39、erchange and is then reinserted at the upstream end of the prover, all automatically. 6.11.2 Sphere interchange may be accomplished with several different combinations of valves or other devices. Each com- bination comprises a system of devices designed to arrest the sphere and pass it through the i
40、nterchange, yet prevent any flow of liquid through the interchange which would bypass the prover section during the proving period. Typical combinations of devices are a) a single special ball valve modified for sphere handling; bl a dual power-operated check valve assembly; CI a combination of a ba
41、ll or gate valve with a power- operated check valve; d a dual through-conduit gate or ball valve; e) a valveless two- or three-sphere assembly; fl an interchange using a plunger-type valve to block the flow. 6.11.3 The controls and actuators used in connection with unidirectional provers will depend
42、 primarily on the degree of automation with which it is desired to operate the proving system. 4 CEN EN*ISO*7278- 2 95 m 3404589 OL3L83b O22 m IS0 7278-2 : 1988 (E) 6.11.4 Separator tees, as shown in figure 1, are sized at least one pipe size larger than the nominal size of the sphere or loop. The d
43、esign of the separator tee shall ensure dependable separa- tion of the sphere from the stream for all flow rates within the flow range of the prover. 6.11.5 Launching tees are generally one pipe size larger than the displacer sphere and shall have smooth transition fittings leading into the prover.
44、The launching tee shall have a slight in- clination downwards toward the prover section, or some other means of ensuring movement of the sphere into the prover dur- ing periods of low flow, such as might occur during calibration by the water draw method. 6.12 Equipment for bidirectional provers 6.12
45、.1 In piston-type bidirectional provers of the design shown in figure 2, the outlets and inlets on the prover ends shall be provided with holes or slots. These shall be deburred and shall have a total area greater than 1,5 times the cross- sectional area of the pipe beyond the outlet. In sphere-type
46、 bidirectional provers with oversized end chambers (see figure 3). the chambers shall be designed so that the displacer cannot obstruct the inlet or outlet openings and thus prevent liquid from flowing. The receiving chambers shall be sized to ensure that the displacer is arrested without shock unde
47、r maximum flow conditions. 6.12.2 A single multiport valve is commonly used for revers- ing the direction of liquid flow, and hence that of the displacer. Other means of flow reversal may also be used. All valves shall allow continuous flow through the meter during proving. The valve size and actuat
48、or shall be selected to minimize pressure drop and hydraulic shock. hi the degree of automation that will be incorporated in the proving operation; i) the size and type of meter that will be proved; j) the facilities that will be required for safely installing and removing the displacer; k) the faci
49、lities that will be required for safely venting and draining the prover. 7.2 Diameter In determining the diameters of the pipes to be used in the con- necting lines, or manifolding, and the prover, the head loss through the pipe prover system shall be compatible with the head loss considered tolerable in the metering installation. Generally, the diameter of the pipe prover and manifolds shall not be less than the outlet diameter of any single meter to be proved. 7.3 Volume In determining the volume of a prover between detectors, the following factors sh
