Correlating a few technical terms 16-01
There are a few technical terms that frequently appear in the
papers I write and the formulas I use. These terms include:
- Specific gravity.
- Specific heat
- Specific speed
- Rpm or speed
If you are going to be messing around with pumps, bearings and
mechanical seals, these words are going to have to become a regular
part of your vocabulary. In this paper I am going to show you how
these words sometimes work together to get us the information we need
to both select and troubleshoot centrifugal pumps and mechanical
seals. We will start with specific gravity; a measure of the weight
of a fluid.
Specific gravity numbers are used to:
- Calculate the pressure if we know the pump's head and to
calculate the head if we know the pressure.
- Calculate NPSH available at the suction of a pump, to
determine if the pump is going to cavitate.
- Calculate the water horsepower generated by the pump, to help
us select the brake horsepower of the motor we need to drive the
Density is a measure of the weight of a fluid. Density is a much
better term than specific gravity and should eventually replace it as
the most popular term. Density is measured in gm/cm3 or
Gravity is used:
- When we use the Bernoulli formula to convert the velocity of
the impeller, or liquid flow to head. Bernoulli told us that the
head in any fluid system is a constant. The head is either in the
form of pressure or velocity head.
Specific Heat determines how hot the product will become with the
addition of heat. The specific heat numbers are used to:
- Calculate the amount of heat generated inside the wet end of
- Determine if the stuffing box temperature will be too
- Calculate the amount of heat generated at the mechanical seal
- Predict if the product will vaporize at the seal face.
- Predict if the hydrocarbon will coke at the seal face or inside the stuffing box.
Specific speed is a dimensionless number that describes the shape
of the pump impeller. We use this number to:
- Determine if our pump is likely to cavitate.
- Select the shape of the pump curve.
- Determine the efficiency of the pump.
- Anticipate motor overloading problems.
- Predict net positive suction head required (NPSHR)
- Select the lowest cost pump for their application.
- To help in the selection of an efficient pump.
- To learn the shape of the pump curve
- To calculate the hydraulic acting on the impeller as we operate off the BEP.
- To determine the direction of shaft deflection if we are
operating off the pump BEP.
Viscosity is defined as resistance to pouring, with higher viscosity
liquids affecting centrifugal pump performance in several ways:
- An increase in horsepower (KW) is needed.
- The head, capacity and pump efficiency will be reduced.
- The mechanical seal will have trouble compensating for shaft
movement and stuffing box misalignment.
- The bearings will be subjected to higher radial loading as the
pump shaft is displaced
- The sealed liquid may not lubricate the lapped faces if the
fluid film thickness is less than 0.000040" (one micron)
Unfortunately there is no acceptable analytical method of
predicting pump performance when the liquid has a viscosity different
than water. Many tests have been conducted and the data formulated
into charts and nomographs with the result being that your pump
performance can be reasonably estimated for liquids of just about any
viscosity. Check with your pump manufacturer for his charts.
Rpm or speed is used:
- To calculate the feet per minute (fpm) at the seal faces to
determine if you need to convert from rotating to stationary seals.Stationary seals should be used at surface speeds
greater than 5000 fpm.
- As a part of the force formula we use to calculate the force
on an impeller when the pump is operating off its BEP.
- In the affinity laws, to determine changes in head, capacity and power
requirements any time you change the speed of the pump, or the
diameter of the impeller.
- To determine the DN number for precision bearings.
- To determine the maximum speed for slurry pumps.
Link to Mc Nally home page www.mcnallyinstitute.com