Page 52 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Installation, Maintenance, and Repair of Horizontal Pumps 37

erances. It is especially important that special types of wear rings be ob-
tained from a knowledgeable producer; typical special parts include ser-
rated, stepped, or reverse-threaded wear rings, bushings, and thrust
balancing devices.
It is always incumbent on process plant management, purchasing, op-
erators and maintenance people to make a conscious effort to minimize
operating costs. One very effective way to accomplish this goal is to be
alert to the adverse effects of excessive wear part clearances in your
pumps.

Calculating the Cost of bur Excess Clearances
Use Figures 1-11, 1-13, and 1-14 to calculate cost of excess wear ring
clearances for the pumps in your plant. Since horsepower losses from
excessive wear ring clearances vary widely by pump type and index, spe-
cific speed (Ns) can be used to simplify these calculations. Use Figure
1-13 and the instructions in step 1 to determine the specific speed for
your pumps.
Step l-the nomogram, Figure 1-13, solves the Ns equation:

Ns = NQ"*/H3"

Where:

Ns = pump specific speed,
N = pump speed in rpm,
Q = pump capacity in gpm,
H = total head per stage in ft.
To determine Ns from this nomogram, draw a line connecting Q and N
for your pump. Note the intersection of this line with the pivot Bine. Then
draw a line connecting this pivot point (the pivot line intersection of the
QN-line) and H; extend the line to the Ns scale and read Ns for the pump.

Step 2-Use Figure 1-11 to determine the percent increase in pump
bhp for the percentage increase in wear ring clearance and N of your
Pump-
Step 3-Determine the normal power requirement of your pump from
the manufacturer's performance curve, or calculate the design horse-
power as follows:
bhp = (QH13960e) (s.g.)