【Q&A Question 160】2017.06.09
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This post was last edited by 955559 on 2017-7-21 23:13. [Q&A Question 160] June 9, 2017: What should be done with the outlet valve when starting a centrifugal pump or a reciprocating pump? Why? The reference answers can be seen after responding; points will be awarded for identifying the key points. (Unless otherwise specified, all questions and answers are based on hydrogenation units.) When starting a centrifugal pump, the outlet valve should be completely closed. Once the motor is running properly, the outlet valve can be gradually opened. The purpose of closing the outlet valve at startup is to prevent the motor from starting under load, which could result in an excessive starting current and damage to the motor. When starting a reciprocating pump, the outlet valve must be opened; this is done to avoid the valve becoming completely closed, which would cause a sharp increase in pressure inside the pump and potentially damage the equipment. Scoring criteria: 2 Wealth points for incorrect responses; *incomplete* answers earn 3–8 points. 10 Wealth points for correct answers, with detailed explanations; responses that are correct but lack sufficient details earn 15–20 Wealth points. Please score according to these criteria. Note: If there are editing records after answering, only points will be awarded to those who participated. For continuous malicious spamming, 2 points will be deducted each time, with no upper limit. Pure numbers, pure emojis, or random letters that have nothing to do with the answer are all considered malicious spamming; points will start to be deducted after three warnings. 2017 Q&A Summary Thread (updates have started) http://bbs.hcbbs.com/thread-1657793-1-1.html 2017 Daily Question Summary Thread (updates have started) http://bbs.hcbbs.com/thread-1657794-1-1.html 2016 Q&A Summary Thread (updates completed) http://bbs.hcbbs.com/thread-1597792-1-1.html
When starting a centrifugal pump, the outlet valve should be completely closed. Once the motor is running properly, the outlet valve can be gradually opened. The purpose of closing the outlet valve at startup is to prevent an increase in the starting current of the motor due to load, which could damage the motor. When starting a reciprocating pump, the outlet valve should be fully open to prevent pressure buildup inside the pump from damaging it.
When the centrifugal pump is started, the outlet valve closes； Protect the pump motor by preventing excessive flow from causing high load on the motor and resulting in overcurrent that can damage it. When the reciprocating pump is started, the outlet valve is fully open to prevent pressure buildup and protect the equipment.
Because the working principles of centrifugal pumps and reciprocating pumps are different. A centrifugal pump expels liquid from the pump through the high-speed rotation of its impeller; the greater the flow rate, the more power is consumed. When the flow rate is zero, the starting load on the motor is reduced. If the outlet valve is not closed, it can lead to vacuum conditions or the motor tripping due to overload. The operation of a reciprocating pump involves forcing liquid out. If the outlet valve or return valve is closed, the pressure at the pump’s outlet rises suddenly, which can damage the pump cylinder, piston, and the outlet piping, thus ruining the pump.
Close the outlet valve of the centrifugal pump to prevent excessive current from damaging the motor during startup and to avoid vacuum formation； Keep the outlet of the reciprocating pump fully open to prevent damage to the pump body
When starting a centrifugal pump, the outlet valve must be closed, whereas for a reciprocating pump, the outlet valve must be opened before starting. The reason why the outlet valve of a centrifugal pump must be closed before starting the pump is that, at the moment the motor starts up during pumping, the resistance on the outlet side is low and the flow rate is high, resulting in the highest current flowing through the pump motor. If the outlet of the pump is open at this time, it is easy for the motor and the electrical wiring to become overloaded due to the high current and heavy load, which can lead to damage. Therefore, starting the pump before opening the valve can reduce the motor’s starting current (the shaft power of a centrifugal pump is lowest when the outlet valve is completely closed), thereby minimizing the impact of startup on the pipeline network. In principle, a reciprocating pump is a type of positive displacement pump; it operates by using a piston or plunger that moves back and forth within the pump cylinder to change the volume of the working chamber, thereby enabling the suction and discharge of liquid. As long as the plunger force and the strength of the working chamber are sufficient, theoretically the pump’s pressure can be infinitely high (ignoring leaks). In reality, the plunger force (provided by the power engine) and the strength of the working chamber (determined by the materials and structure) are always limited; therefore, if the pressure keeps increasing, the result is either the destruction of the power engine or the damage to the working chamber. Therefore, before starting the pump, all the discharge valves in the piping system must be opened; it is not allowed for the discharge lines to become blocked, as this could lead to equipment damage or injuries. To ensure safety, a safety valve must also be installed on the pump’s discharge line, so that the discharge pressure does not exceed its rated value. This means that, during startup and operation, reciprocating pumps operate in the opposite way to centrifugal pumps – before starting a centrifugal pump, the discharge valves in the piping system must be closed, thereby minimizing the starting power required. This is what we usually refer to as open-starting for reciprocating pumps and closed-starting for centrifugal pumps.
Close the centrifugal pump and turn on the reciprocating pump to prevent large voltage spikes; turning it on helps avoid pressure buildup
The outlet valve should be closed when starting the centrifugal pump, to prevent excessive current, tripping, or damage to the electrodes! A reciprocating pump is a positive-displacement pump; the outlet valve should be opened when starting the pump to prevent damage to the pump due to excessive pressure!
When starting a centrifugal pump, the outlet valve must be closed, whereas for a reciprocating pump, the outlet valve must be opened before starting； Centrifugal pumps generally regulate flow rate through the outlet valve, while reciprocating pumps adjust flow rate by controlling the stroke.
When a centrifugal pump starts, the outlet valve is fully closed; when a reciprocating pump starts, the outlet valve is fully open
When starting a centrifugal pump, the outlet valve should be completely closed; once the motor is running properly, the outlet valve can be gradually opened. The purpose of closing the outlet valve at startup is to prevent the motor from starting under load. When starting a reciprocating pump, the outlet valve should be fully open to avoid pressure buildup at the outlet.