Three stages and 10 steps in the operation of a centrifugal pump

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The stability management of pumps in the process industry can be divided into three stages: 1) Pump selection and determination of stability prior to installation； 　　2) Preparation for startup ； 　　3) Ensure stability after startup. For a centrifugal pump to operate more efficiently and stably, it is necessary to take proper care in the following 10 areas. I. Pump Selection As long as the centrifugal pump operates under conditions close to those for which it was designed, it can generally achieve good performance and a long maintenance interval. It should be noted, however, that \"design\" refers not only to pressure, flow rate, temperature, and other process parameters, but also to the pressure and movement of flanges, the pressure and movement of couplings, the lubrication of bearings, and similar mechanical factors. Pumps with outdated designs can only operate for a short period under conditions other than those for which they were designed, after which their performance levels decline. Therefore, a pump that operates near the best efficiency point (BEP) should be selected. 　　Centrifugal pumps with a high suction specific speed can usually achieve good operating conditions only within a narrow range. Backflow occurs in high-head pumps that attempt to use a large impeller inlet to reduce NPSHR. 　　The suction specific speed of pumps with a low NPSHR is around 12,000. If the normal flow rate deviates from the BEP flow rate, internal backflow occurs, thereby reducing the operational stability of the pump. Turbulence can cause corrosion of the impeller, as well as mechanical sealing issues, bearing loads, and shaft deflection. The expected service life of centrifugal pump components is influenced by many factors, including the pump’s suction specific speed, flow percentage (Qactual/QBEP), NPSH margin (NPSHA-NPSHR), head increase per stage, and pump casing design. The user’s demand for efficient, low-cost pumps with a low NPSHR has led to an inlet specific speed of over 12,000, at which point the deviation of the actual flow rate from the BEP flow rate is small. The ideal suction specific speed for the pump should be around 8,500. II. Selection of Seals Packing seals have fallen out of use; mechanical seals and dry gas seals are now widely employed, with mechanical seals being the most common choice. Mechanical seal products and technologies are highly mature. The material used for the sealing surface must be able to dissipate heat generated quickly, thereby preventing the fluid from evaporating. Silicon carbide is the preferred material for sealing end faces due to its high thermal conductivity and high hardness. The sealed flushing scheme needs to be carefully considered when selecting the pump. III. Outlet Pressure When the pipe is connected to the pump casing flange via bolts, the external pressure resulting from the weight of the pipe, thermal expansion, and the weight of the process fluid all exert pressure on the pump. These pressures will cause the pump casing to deform, leading to internal misalignment and resulting in friction between the pump casing, the impeller, and the shaft, which may even cause the shaft to stop rotating. They generate torque around the pump, causing a misalignment between the pump and the drive shaft, which in turn leads to premature failure of the coupling, bearings, or mechanical seal. Therefore, limits must be set for the pressure and torque of these ports. For pipeline designers, these limits represent the maximum pressure values that a pipeline can exert on a pump. IV. Storage Once the pump has been properly designed and ordered, the seller will deliver it to the buyer for installation. All pumps are covered with waterproof tarpaulins, ensuring that the tarpaulins fully cover both the top and bottom of the pumps in order to prevent dust from getting into the bearings and couplings. Place the pump in a rainproof and dustproof location to prevent dust accumulation and moisture. V. Pump location The correct positioning of the pump is important for both operating and maintenance purposes. To ensure good flow conditions, the pump should be placed as close as possible to the liquid supply end. If there is enough space, the pump inlet should be below the level of the liquid being supplied. Sufficient space should be left around the pump to facilitate inspection and maintenance. Sufficient space should be left at the top of the pump to allow a overhead crane and a chain crane to lift the heaviest parts of the pump. 　　Install the pump in a clean and dry place; try to avoid dirty, dusty, or humid environments. VI. Foundation The foundation of the pump must be carefully designed to ensure that the loads on the pump and its drive mechanism do not exceed the bearing capacity of the soil. Almost all pumps are installed on concrete foundations, as this material is inexpensive and very strong. VII. Pump Installation One of the most critical factors for the successful operation and maintenance of pumps of all sizes and types is proper installation. A properly installed pump can remain in its calibrated position for a long time, with less leakage from the pump casing and flanges, as well as reduced vibration. VIII. Pipeline Connection The loads and stresses applied to the pipeline system can **affect the stability of the equipment**. Loads caused by pipe expansion or other factors can lead to shaft deflection, deformation of the pump casing, and interference between the internal rotating components. Therefore, the less stress the piping system exerts on the equipment, the better. IX. Cleaning of pipes
There are gaps between the rotating components of the pump; it is essential to prevent abrasive particles in the new piping system from causing damage to them. Abrasive particles that accumulate in the rotating clearances of the pump can cause the pump to shut down, resulting in very high maintenance costs. To reduce the likelihood of abrasive particles entering the pump, filters are generally installed. X. Calibration – Bring the pump to its operating temperature by introducing liquid into the pump casing. When checking the calibration, the rotation of the motor and pump should be inspected. The pump should rotate in the direction of the arrow on the pump casing. The offset may cause: 1) Overload of the pump bearings ； 　　2) Axial rotation before and after the mechanical seal: the more the seal moves, the easier it is for the overlapping seal surfaces to separate ； 　　3) If the offset is severe, the stationary and moving parts will come into contact ； 　　4) Wear ring contact ； 　　5) The impeller and volute are in contact with each other.