API MPMS 4.9.2 2005 pdf download.Manual of Petroleum Measurement Standards Chapter 4—Proving Systems Section 9—Methods of Calibration for Displacement and Volumetric Tank Provers Part 2—Determination of the Volume of Displacement and Tank Provers by the Waterdraw Method of Calibration.
5.3 PURGING AND CLEANING OF THE PROVER Provers in batched crude oil service should be purged with the least viscous available crude unless there is on-site availability of other light hydrocarbon products that can be used. Regardless of service the prover and associated piping shall be cleaned by the best means available and flushed with water until free of all traces of hydrocarbon liquids and solids. Valves may need to be oper- ated to ensure that no hydrocarbon remains. The most widely used method is running the prover displacer back and forth during the water-flush to scrub the walls. Another method, used in crude oil service is to use a hot oil truck to circulate hot water through the prover while pushing the displacer back and forth. When available, steam cleaning is a method for removing wax, paraffin, etc. Pull-through foam pipeline pigs can also be used as an alternative or an additional method of cleaning the interior of pipe provers. Caution should be taken to prevent damage to the internal coating during the cleaning process. While a detergent can be effective in cleaning a prover, all traces of it must be removed prior to the calibration to prevent foaming. This can sometimes be very difficult to achieve. Therefore, the use of detergents is discouraged except in extreme cases. If it is decided that a detergent has to be used, only a non-suds type should be considered. Even then, it may take a great amount of water to fully remove all traces of the detergent. Nonetheless, all traces of the presence of any detergent must be totally removed before beginning the waterdraw calibration. LPG pipe provers are best cleaned by filling with water and moving the displacer back and forth to scrub the walls. It is recom- mended that the prover be drained, refilled and then allowed to stand overnight. The prover is then drained and flushed with clean water.
5.4 SPHERE DISPLACER INSPECTION, SIZING AND PREPARATION Prior to inspection the prover sphere has to be removed from the prover. In order to remove the sphere displacer, send it to the appropriate end chamber if the prover is bi-directional, or to the sphere handling interchange if the prover is unidirectional. Then drain the prover and remove the access cover. Provided the sphere is in place it can be removed either by hand or mechanical means depending upon its size. Sometimes it will be found that the sphere has not in fact arrived in the chamber but is further back in the prover barrel. If this happens only one procedure is permissible: replace the cover securely, refill the prover, and start again. Under no circumstances shall any attempt be made to force the displacer out with compressed air or gas. Once the prover sphere has been removed, check the condition to make sure it does not have deep cuts, rips, extreme pitting, holes, flat spots, soft spots, or overall poor condition that may cause leaks or loss of seal. After careful inspection, a decision on its suitability for further use should be made. The durometer (hardness) and composition of the sphere should be considered as part of prover design. Different hardness and materials can have an effect on both normal operations and calibration, so these must be considered in the design to accommodate both. Because of the lower lubricity of water and greatly reduced flow rates encountered during waterdraw calibrations, a softer durometer sphere usually has a better performance during calibrations. Harder durometer spheres have a difficult time creating a capillary seal inside a prover with anything less than excellent interior coating and round pipe. Conversely, caution should be taken to ensure that the durometer of the sphere is not too soft, otherwise it might have a hard time compressing the spring on the detector switch probe.
Verify the ovality or roundness of the sphere by determining its circumference around two separate axes perpendicular to each other. A circumference variation in the sphere, that is the difference in length around these two perpendicular axes of more than one percent of the nominal circumference, is considered out-of-round. Measuring the sphere, first around its equator, and then around its polar axis usually across the two valve holes, and comparing the difference between the two measurements according to Table 1 will verify sphere roundness. Manufacturing tolerances for new spheres may be up to 1.5%. Examples of Sphere Ovality Verification: A 6 in. sphere that is considered to be round, with a nominal diameter of 6.065 in. would be expected to have a diameter variation no greater than 1 / 16 in. and a circumference variation no greater than 3 / 16 in. Alternatively, a 30 in. sphere with a nominal diameter of 29.250 in. would be expected to have a diameter variation no greater than 9 / 32 in. and a circumference variation no greater than 29 / 32 in. The comparison measurements in all cases shall be taken around two perpendicular axes of the sphere.