This is an important part of the pump system and should be thought of as a very specialized piping design. Considerable attention must be directed to the pump suction piping to ensure satisfactory pump operation.
A pump is designed to handle liquid, not vapor, except possibly some vapor entrained or absorbed in the liquid. The liquid or its gases must not vaporize in the eye/entrance of the impeller. (This is the lowest pressure location in the impeller.) Unfortunately, for many situations, it is easy to get vapor into the pump if the design is not carefully done. Vapor forms if the pressure in the pump falls below the liquid’s vapor pressure. The lowest pressure occurs right at the impeller inlet where a sharp pressure dip occurs. The impeller rapidly builds up the pressure, which collapses vapor bubbles, causing cavitation and damage. This must be avoided by maintaining sufficient net positive suction head (NPSH) as specified by the manufacturer.
Net positive suction head (in feet (m) of liquid absolute) above the vapor pressure of the liquid at the pumping temperature is the absolute pressure available at the pump suction flange, and is a very important consideration in selecting a pump which might handle liquids at or near their boiling points, or liquids of high vapor pressures.
Do not confuse NPSH with suction head, as suction head refers to pressure above atmospheric. If this consideration of NPSH is ignored the pump may well be inoperative in the system, or it may be on the borderline and become troublesome or cavitating. The significance of NPSH is to ensure sufficient head of liquid at the entrance of the pump impeller to overcome the internal flow losses of the pump. This allows the pump impeller to operate with a full “bite” of liquid essentially free of flashing bubbles of vapor due to boiling action of the fluid.
The pressure at any point in the suction line must never be reduced to the vapor pressure of the liquid. Both the suction head and the vapor pressure must be expressed in feet (m) of the liquid, and as gauge pressure or absolute pressure.
For low NPSHA (available) (less than 10 ft or 3 m) the pump suction connection and impeller eye may be considerably oversized when compared to a pump not required to handle fluid under these conditions.
Poor suction condition due to inadequate NPSHA is one major contribution to cavitation in pump impellers, and this is a condition at which the pump cannot operate for very long without physical erosion damage to the impeller.
The simple equation to get NPSH value is :
NPSH = (pressure head at the source) + (static suction head) - (friction head in the suction line) - (vapor pressure of the liquid).
Recommendations also are made by the Hydraulic Institute of suction specific speeds for multistage boiler feed pumps, with S = 7900 for single suction and S = 6660 for double suction. Thus the required NPSH can be found by equation :
NPSH = [(rpm)(gpm)0.5/S]4/3
For example, at 3500 rpm, l000 gpm, and S = 7900, the required NPSH is 34 ft
Therefore, the suction system must perform two major jobs: maintain sufficient NPSH; and maintain the pressure above the vapor pressure at all points.
NPSH is the pressure available at the pump suction nozzle after vapor pressure is subtracted. It is expressed in terms of liquid head. It thus reflects the amount of head loss that the pump can sustain internally before the vapor pressure is reached. The manufacturer will specify the NPSH that his pump requires for the operating range of flows when handling water. This same NPSH is normally used for other liquids.
For design work, the known pressure is that in the vessel from which the pump is drawing. Therefore, thepressure and NPSH available at the pump suction flange must be calculated. The vessel pressure and static head pressure are added. From this must be subtracted vapor pressure and any pressure losses in the entire suction system such as:
1. Friction losses in straight pipe, valves, and fittings
2. Loss from vessel to suction line
3. Loss through equipment in the suction line (such as a heat exchanger)
The NPSH requirement must be met for all anticipated flows. Maximum flow will usually have a higher NPSH than normal flow. For some pumps, extremely low flows can also require higher NPSH.
It is usually necessary for the process engineer to have an idea of NPSH requirements early in the design phase of a project. The NPSH sets vessel heights and influences other design aspects. The choice of pumps is an economic balance involving NPSH requirements and pump speed. The lower speed pump will usually have lower NPSH requirements and allow lower vessel heights. A low-speed pump may also have a better maintenance record. However, the higher-speed pump will usually deliver the required head in a cheaper package.
The suction system piping should be kept as simple as reasonably possible and adequately sized. Usually the suction pipe should be larger than the pump suction nozzle.
A pump is designed to handle liquid, not vapor, except possibly some vapor entrained or absorbed in the liquid. The liquid or its gases must not vaporize in the eye/entrance of the impeller. (This is the lowest pressure location in the impeller.) Unfortunately, for many situations, it is easy to get vapor into the pump if the design is not carefully done. Vapor forms if the pressure in the pump falls below the liquid’s vapor pressure. The lowest pressure occurs right at the impeller inlet where a sharp pressure dip occurs. The impeller rapidly builds up the pressure, which collapses vapor bubbles, causing cavitation and damage. This must be avoided by maintaining sufficient net positive suction head (NPSH) as specified by the manufacturer.
Net positive suction head (in feet (m) of liquid absolute) above the vapor pressure of the liquid at the pumping temperature is the absolute pressure available at the pump suction flange, and is a very important consideration in selecting a pump which might handle liquids at or near their boiling points, or liquids of high vapor pressures.
Do not confuse NPSH with suction head, as suction head refers to pressure above atmospheric. If this consideration of NPSH is ignored the pump may well be inoperative in the system, or it may be on the borderline and become troublesome or cavitating. The significance of NPSH is to ensure sufficient head of liquid at the entrance of the pump impeller to overcome the internal flow losses of the pump. This allows the pump impeller to operate with a full “bite” of liquid essentially free of flashing bubbles of vapor due to boiling action of the fluid.
The pressure at any point in the suction line must never be reduced to the vapor pressure of the liquid. Both the suction head and the vapor pressure must be expressed in feet (m) of the liquid, and as gauge pressure or absolute pressure.
For low NPSHA (available) (less than 10 ft or 3 m) the pump suction connection and impeller eye may be considerably oversized when compared to a pump not required to handle fluid under these conditions.
Poor suction condition due to inadequate NPSHA is one major contribution to cavitation in pump impellers, and this is a condition at which the pump cannot operate for very long without physical erosion damage to the impeller.
The simple equation to get NPSH value is :
NPSH = (pressure head at the source) + (static suction head) - (friction head in the suction line) - (vapor pressure of the liquid).
Recommendations also are made by the Hydraulic Institute of suction specific speeds for multistage boiler feed pumps, with S = 7900 for single suction and S = 6660 for double suction. Thus the required NPSH can be found by equation :
NPSH = [(rpm)(gpm)0.5/S]4/3
For example, at 3500 rpm, l000 gpm, and S = 7900, the required NPSH is 34 ft
Therefore, the suction system must perform two major jobs: maintain sufficient NPSH; and maintain the pressure above the vapor pressure at all points.
NPSH is the pressure available at the pump suction nozzle after vapor pressure is subtracted. It is expressed in terms of liquid head. It thus reflects the amount of head loss that the pump can sustain internally before the vapor pressure is reached. The manufacturer will specify the NPSH that his pump requires for the operating range of flows when handling water. This same NPSH is normally used for other liquids.
For design work, the known pressure is that in the vessel from which the pump is drawing. Therefore, thepressure and NPSH available at the pump suction flange must be calculated. The vessel pressure and static head pressure are added. From this must be subtracted vapor pressure and any pressure losses in the entire suction system such as:
1. Friction losses in straight pipe, valves, and fittings
2. Loss from vessel to suction line
3. Loss through equipment in the suction line (such as a heat exchanger)
The NPSH requirement must be met for all anticipated flows. Maximum flow will usually have a higher NPSH than normal flow. For some pumps, extremely low flows can also require higher NPSH.
It is usually necessary for the process engineer to have an idea of NPSH requirements early in the design phase of a project. The NPSH sets vessel heights and influences other design aspects. The choice of pumps is an economic balance involving NPSH requirements and pump speed. The lower speed pump will usually have lower NPSH requirements and allow lower vessel heights. A low-speed pump may also have a better maintenance record. However, the higher-speed pump will usually deliver the required head in a cheaper package.
The suction system piping should be kept as simple as reasonably possible and adequately sized. Usually the suction pipe should be larger than the pump suction nozzle.
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