High-pressure feed water pump (reciprocating pump) inlet and outlet cross-piping for hydrocracking
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I have seen that on the inlet and outlet of the injection pumps (reciprocating pumps) in many devices, there is a cross-connected line. What is the specific function of this cross-connected line? Is it only meant to be used as a pump for trial operation in standalone mode, or is it also used to balance the inlet and outlet pressures in tank pumps? I’ve seen some systems use this cross-line to adjust the outlet flow rate when starting the pump, that is, to serve as a minimum flow line. But I don’t think that’s correct, because it doesn’t function as a minimum flow line – it doesn’t return from the outlet to the tank but rather directly back to the inlet pipeline. If this is used to adjust the outlet flow rate after the pump starts, the high pressure at the outlet will flow back to the inlet via this shortcut, which will inevitably cause overpressure at the inlet. So I’d like to discuss with everyone what the purpose of designing this cross-line is I feel that it doesn’t play a significant role when the pump is running, and I think it should definitely not be used to control the outlet flow while the pump is in operation. Please share your own opinions as well.
⒈For use as a stand-alone trial pump； ⒉There is a return line for the outlet safety valve.
Only high-speed centrifugal pumps have a minimum flow line; yours is a reciprocating pump. Safety valves are generally installed at the outlet of reciprocating pumps, and this line represents the return line for those safety valves, similar to the vent line of a container’s safety valve.
Return line of the outlet safety valve? What does that mean? I don’t quite understand! Could you explain this one?
Is the line you’re referring to the one used when starting the pump? I think this line should not be used after the pump is started; it’s likely intended for balancing the pressure at the inlet and outlet during pump operation
Hello, may I ask what you mean by the return line for the outlet safety valve? Could you explain it?
Generally, a safety valve is installed at the pump outlet; this valve releases pressure back to the pump outlet to prevent the reciprocating pump from building up excessive pressure. It is not the minimum flow line; it must never be used to adjust the flow rate in order to prevent cross-contamination between high and low pressures. As mentioned on the 2nd floor, this cable can be used when running in standalone mode.
High-pressure pumps have a minimum flow rate limit; the hydrocracking process requires frequent adjustment of the high-pressure water injection based on its processing capacity and the nitrogen content of the feedstock, hence cross-connections at the inlet and outlet of the high-pressure water injection pump are used to regulate the water volume.
The return line from the outlet to the inlet is used to regulate the water injection volume. The working mechanism of a reciprocating pump differs from that of a centrifugal pump; therefore, it is not possible to install a control valve at the outlet (as this would cause excessive pressure at the outlet of the reciprocating pump, triggering the safety valve). Thus, regulating the flow rate can only be achieved by controlling the amount of fluid returning
I’ve seen a lot of materials that are used to adjust the water injection volume! Type: Centrifugal pump, Reciprocating pump
Flow rate: 1. Uniform 2. High volume 3. Flow rate varies depending on the pipeline conditions 1. Non-uniform 2. Low volume 3. Constant flow rate, hardly affected by changes in head
Head: 1. Generally not high 2. Only a certain head can be provided for a given flow rate 1. High 2. Different heads can be supplied for a given flow rate, determined by the piping system
Efficiency: 1. Around 70% 2. Highest at the design point; efficiency decreases as deviation increases 1. Around 80% 2. Remains high even when different heads are supplied
Structure: 1. Simple, inexpensive, easy to install 2. Rotates at high speed, can be directly connected to a motor 3. Small volume for the same flow rate 4. High requirements for shaft sealing to prevent leaks 1. Many parts, complex structure 2. Severe vibration, cannot operate at high speeds, difficult to install 3. Large size, occupies much space 4. Requires suction and discharge valves 5. More complex structure when transporting corrosive liquids
Operation: 1. Needs to be flushed before operation; no leaks during operation 2. Easy to maintain and operate 3. Flow rate can be easily adjusted using valves 4. No damage caused by pipeline blockages 1. Many parts, prone to failure, difficult to repair 2. Flow rate can only be adjusted using bypass valves, not outlet valves 3. Maintains high efficiency even when head and flow rate change
Application range: Can transport corrosive substances or suspensions; not suitable for liquids with high viscosity. Generally has high flow rates but low head. Suitable for clean liquids requiring high head and low flow rate. Positive displacement pumps cannot have their flow rate adjusted by closing the discharge valve, as such adjustment is ineffective, wastes energy, and may even overload the pump’s drive mechanism. From the characteristic curve of the reciprocating pump, it can be seen that: ① the theoretical flow rate QT is independent of the discharge pressure P (but as the discharge pressure increases, internal leakage in the pump increases, resulting in a slight decrease in the actual flow rate of the pump). ②The shaft power N of positive displacement pumps increases as the discharge pressure rises. ③The efficiency of positive displacement pumps increases as the discharge pressure rises. For electric reciprocating pumps, a bypass control valve is generally installed to regulate the flow rate. The excess liquid from the pump is returned to the suction pipe via a bypass. For electric proportional pumps and metering pumps, the flow rate of the pump is usually adjusted by changing the shape of the piston or plunger. For steam reciprocating pumps, the number of reciprocations of the pump is usually adjusted by controlling the opening degree of the steam inlet valve, thereby regulating the flow rate.
Flow rate: 1. Uniform 2. High volume 3. Flow rate varies depending on the pipeline conditions 1. Non-uniform 2. Low volume 3. Constant flow rate, hardly affected by changes in head
Head: 1. Generally not high 2. Only a certain head can be provided for a given flow rate 1. High 2. Different heads can be supplied for a given flow rate, determined by the piping system
Efficiency: 1. Around 70% 2. Highest at the design point; efficiency decreases as deviation increases 1. Around 80% 2. Remains high even when different heads are supplied
Structure: 1. Simple, inexpensive, easy to install 2. Rotates at high speed, can be directly connected to a motor 3. Small volume for the same flow rate 4. High requirements for shaft sealing to prevent leaks 1. Many parts, complex structure 2. Severe vibration, cannot operate at high speeds, difficult to install 3. Large size, occupies much space 4. Requires suction and discharge valves 5. More complex structure when transporting corrosive liquids
Operation: 1. Needs to be flushed before operation; no leaks during operation 2. Easy to maintain and operate 3. Flow rate can be easily adjusted using valves 4. No damage caused by pipeline blockages 1. Many parts, prone to failure, difficult to repair 2. Flow rate can only be adjusted using bypass valves, not outlet valves 3. Maintains high efficiency even when head and flow rate change
Application range: Can transport corrosive substances or suspensions; not suitable for liquids with high viscosity. Generally has high flow rates but low head. Suitable for clean liquids requiring high head and low flow rate. Positive displacement pumps cannot have their flow rate adjusted by closing the discharge valve, as such adjustment is ineffective, wastes energy, and may even overload the pump’s drive mechanism. From the characteristic curve of the reciprocating pump, it can be seen that: ① the theoretical flow rate QT is independent of the discharge pressure P (but as the discharge pressure increases, internal leakage in the pump increases, resulting in a slight decrease in the actual flow rate of the pump). ②The shaft power N of positive displacement pumps increases as the discharge pressure rises. ③The efficiency of positive displacement pumps increases as the discharge pressure rises. For electric reciprocating pumps, a bypass control valve is generally installed to regulate the flow rate. The excess liquid from the pump is returned to the suction pipe via a bypass. For electric proportional pumps and metering pumps, the flow rate of the pump is usually adjusted by changing the shape of the piston or plunger. For steam reciprocating pumps, the number of reciprocations of the pump is usually adjusted by controlling the opening degree of the steam inlet valve, thereby regulating the flow rate.
The inlet and outlet crosslines of the reciprocating injection pump are used for starting and stopping the pump, enabling start-up and shutdown at zero load.
1. Start-up and shutdown with no load; 2. Can be used to regulate flow rate.