INTRODUCTION
Sand production in Oil and Gas Separators possess various adverse effects on Separation System. High sand rate may disrupt equipment performance, clogging, pipe erosion, process upsets and etc. In such case, a Sand Jetting System is integrated online alongside a jet pump to maintain minimal sand residue inside separators.
A conventional Sand Jetting System tends to introduce turbulence/ disturbance in separator. The turbulence are generated by driving motive fluid to a set of nozzles being positioned at the bottom of separator. This causes separation performance to degrade during jetting process. Another superior solution is to introduce cyclonic jetting nozzle that blasts the water horizontally to create vortex underneath the separator for sand suction. A decent approach to apply cyclonic jetting nozzle is by introducing a jet pump at its discharge line. Jet pump is also known as ejector and eductor, depending on the usage. It is normally adopted to generate sufficient pressure difference in between sand slurry at the inside of separator and sand disposal system for sand carriage. For pressurized tank, jet pump usage is optional, whereas for atmospheric tank, jet pump is required to lower outlet pressure. The combination of both cyclonic jetting nozzle and jet pump is ideal for effective sand removal in oil and gas separator.
DESCRIPTION
1.0 Cyclonic Jetting Nozzle
A set of sand jetting nozzles are introduced at the bottom of separator for transportation of sand slurry onto sand disposal system. These nozzles consist of swirling chamber connected to motive pipe that is being offset to create swirl effect, alongside discharge pipe in the middle of it. Swirl chamber spins motive fluid to create spiral fluid motion at bottom of each nozzles. At the bottom of each nozzle, there is nozzle opening for circular water spray designed to minimize turbulence generated on top of the separation layer. As a result, oil and water separation is maintained throughout sand jetting procedure. The vortex created will lift sand particles and low-pressure discharge pipe created by jet pump will eventually suck the lifted slurry mixture to be dumped overboard. The position of cyclonic jetting nozzle is shown in Figure 1.
The selection of cyclonic jetting nozzle depends on maximum particle sizes and required mass flow rate of solids. Our in-house design, however, is able to provide low sand residue by having additional modification on nozzle opening and improved swirling chamber. In Figure 2, it is shown that our in-house cyclonic jetting nozzles offer wider coverage area and cleaner sand residue as compared to conventional design. Based on those generic operating data, cyclonic jetting nozzles are superior than normal sand jetting system.
2.0 Jet Pump for Sand Removal
The use of jet pump coupled with cyclonic jetting nozzles is ideal for competent sand transport at pipe discharge. Suction piping of the jet pump is attached to the discharge pipe of cyclonic jetting nozzle, and the jet pump needs to be designed in such a way that its throat pressure is lower than the incoming slurry pressure for smooth operation. Jet pumps typically possess narrow operating envelope, which makes it mandatory to properly design critical dimensions including its nozzle size, throat diameter and diffuser profile. A jet pump makes an ideal solution for multiphase sand jetting transport deriving from the fact that the suction phase, viscosity and volume fraction are flexible as long as suction rate is met, though a proper process condition would be required to be met for the jet pump to operate at its best.
Jet pump performance is generally defined by either entrainment ratio, the ratio of suction mass rate to motive mass rate; or compression ratio, the ratio of discharge pressure to suction pressure. These variables determine the effectiveness of motive fluid entraining suction fluid by means of fluid mixing, and the total pressure boosting that the jet pump is able to deliver. In some cases, jet pump is designed for pressure boosting, but more often, it is for suction application as demonstrated in sand removal system. For sand removal, it is considered as liquid-multiphase system that perform almost identical to liquid-liquid jet pump except for the density and volume fraction variations. Even so, jet pump is more compact than other pumping solution and can operate without moving parts, thus keeping low maintenance cost. However, there are still drawbacks of using jet pump such as high motive rate required to operate, limited to specific process condition and low entrainment ratio for liquid-multiphase jet pump. Nonetheless, for cases where separator operates at pressure close to its upstream equipment, jet pump can assist sand movement by lowering pressure at discharge side of cyclonic jetting nozzle. For sand removal, such condition can be formed by pushing motive water into a restricted orifice, thus higher velocity and low-pressure region is created downstream the nozzle. As a result, high velocity liquid is channeled towards jet pump throat and suction slurry will be pushed towards jet pump discharge due to differential pressure. While jet pumps are well suited for slurry transport, its high erosion rate may be a challenge for the application of sand removal system, especially the throat section of a jet pump. A proper selection of external coatings and erosion resistant materials is essential to sustain the internal jet pump profile for optimum operation. Another concern is the nozzle, similarly with possible erosion, as well as issue with potential sand existence at motive line, or its opening being enlarged causing lower velocity generated at throat sector that might have a negative impact to the performances of system.
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