Pump selection

When Specific Gravity and Viscosity differ from those of Water

Correction of the head, flow rate and power required (kW) in accordance with the viscosity and specific gravity is required.
As a general rule, even when the specific gravity of the liquid differs from that of water, total head remains the same as for the case of water, as long as the discharge pressure produced by the pump is indicated in liquid column. However, in actual practice, the pump characteristics will change slightly and the capacity-head curve will become erroneous.
In case of specific gravity of the liquid to be considerably smaller than 1.0, though actual operation with such liquid requires smaller motor, operation with water requires the corresponding motor size. Therefore, it is necessary to consider the case of shop test with water when deciding motor size.

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NPSH

NPSH is the abbreviation for Net Positive Suction Head. If the minimum suction head required by the pump is not obtained, cavitation will occur and the pump cannot handle the liquid or if it can handle it, it will be unstable. Since the pump may also suffer considerable damage, either a pump which satisfies these conditions must be selected in accordance with the following equation or make the installation conditions to match with it.

NPSH数式

  • NPSHavail. ≧ 1.3 × NPSH required must prevail to provide a margin.
  • Absolute pressure on liquid surface of suction   vessel (MPa abs.)
  • γ = Specific gravity of liquid (kg/liter)
  • hs = Height from center line of pump inlet to liquid   level of suction vessel (m)
    Higher than center (+)
    Lower than center (-)
  • hsl = Total suction side piping loss (m)
  • Pv = Vapor pressure of liquid at pump suction port   temperature (MPa abs.)
  • g = Gravitational acceleration of 9.80665m/s2
  1. As well as the total head, NPSH required is represented in meters (m). This NPSH required is given also in meters (m) of liquid column. As long as the NPSH available is given in meters (m), it is not necessary to divide by the specific gravity.
  2. The major point to note in the case of the Non-Seal Pump is that, with the basic type model, the liquid which returns to the pump chamber from the rotor chamber cools the motor and, as a result, a temperature rise of about 1-3°C occurs.
    Therefore, Pv in the above equation must be calculated not only with the pump inlet temperature but also with the suction port temperature plus temperature rise when liquid is passing through the rotor chamber.
    This temperature rise can generally be disregarded, however, in the case of saturated liquified gas, a construction like the Reverse Circulation Type is selected which will not be affected by temperature rise.
  3. To satisfy a lower available NPSH condition, to equip an inducer is systematized in the Non-Seal Pump. Since the same models of the pump can offer NPSH required even below 0.5 meter, please consult us when a condition should be critical.

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Temperature

Selection of the type basing on the temperature is mainly made by the class of insulation, the degree of ex-proof protection and the arrangement of cooling jacket on the canned motor.
Liquid temperature limits given are for pumping water or equivalent liquid, and when the specific heat and heat conduction factor of the liquid is poor, it is necessary to deem this limit to be lower by 5-10°C.
For liquid temperatures of -40°C or below, use insulation Class C.

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Vapour Pressure

Since the Non-Seal pump contains no dynamic seal, it is ideal for the handling of liquids having a high vapour pressure or of liquified gases.
However, the following precautions should be observed.

  1. Cooling the motor itself with a stator jacket will be required if there may be the case of the considerable drop in the NPSH due to the temperature rise of the liquid passing through the rotor chamber.
  2. Cooling water, low temperature brine, or other cooling liquids must be used in accordance with conditions. However, there are also cases that the motor is cooled with latent heat by passing a part of the pumped liquid vapourized through a jacket.
    In both cases, extreme care if required to insure that cooling system operation does not stop.
  3. Reverse circulation is one method of insuring that the temperature rise, which occurs when the liquid passes through the inside of the rotor chamber, does not cause a drop in the NPSH.
    The circulation liquid passes through the rotor chamber from the pump chamber and then returns to the vapour zone of suction vessel from rear part of pump.
    For details, refer to paragraph "Reverse circulation piping" on the 4.2.3.

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Motor

3-phase motor is used for the canned motor of the Non-Seal Pump and is fundamentally similar to the ordinary induction motor.

However by "canning";

  1. When the revolving magnetic flux, which rotates the rotor, passes through the stationary can, eddy current will develop in the can. (mainly in the stator can and very little in the rotor can).
    This eddy current will be accompanied by excessive heat but this value will be settled by the specific resistance of can material. Standard material of can is SUS316L (18Ni-12Cr-2.5Mo low carbon stainless steel) or HC.
  2. There are no cooling fans for the motor and cooling is performed by the liquid handled. By recirculating a part of the pumping liquid, both cooling of the motor and lubricating of the bearings are performed.
    If the temperature of liquid handled is high, motor winding of higher insulation class (Class F or Class C) is selected or a jacket or heat exchanger is installed around the outer surface of the stator.
    Temperature limits of motor winding of Non-Seal Pump are as follows:
    Table 2-1 Thermostat Activation Temperature
    Insulation Thermostat Activation
    Temperature
    Class F 155 ± 6 °C
    Class C 224 ± 9 °C
    The maximum load carrying capacity of the thermostat is as follows:
    Voltage: 220 VAC, 50 to 60 Hz
    Current: 1.5 A
  3. Explosion Proof (Flame proof) authorization by RIIS (Research Institute of Industrial Safty, Ministry of Labor Japan) has been acquired for Non-Seal Pumps.
  4. One of the main features of the Non-Seal Pump is that all motors are connectable to the pump of each model. This simplifies maintenance and specification changes.
    Table 2-2 shows a possible combination of pump No. and canned motor.
    The motors between the thick vertical lines indicate that outside diameter are identical except the lengths.
    In other words, in this section, bearings, shaft sleeves, thrust washers etc. wearing parts are interchangeable for easy maintenance. The parts printed in black show pumps and motors combined mutually.

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