MTR(E), MTC, MTA - Grundfos

2
MTC
70
Minimum inlet pressure - NPSHR
Calculation of the inlet pressure "H" is recommended
when ....
• the liquid temperature is high,
• the flow is significantly higher than the rated flow,
• inlet conditions are poor.
To avoid cavitation, make sure that there is a minimum
pressure on the suction side of the pump. The
maximum suction lift "H" in feet of head can be
calculated as follows:
H = pb – NPSHR – Hf – Hv – Hs pb = Barometric pressure in feet absolute.
(Barometric pressure can be set to 33.9
feet).
In closed systems, pb indicates the system
pressure in feet.
NPSHR = Net Positive Suction Head in feet of head.
(To be read from the NPSHR curve at the
highest flow rate the pump will be
delivering).
H f
H v
= Friction loss in suction pipe in feet of head.
(At the highest flow rate the pump will be
delivering.)
= Vapor pressure in feet.
(To be read from the vapor pressure scale.
"H v " depends on the liquid temperature
"T m ").
Hs = Safety margin = minimum 2.0 feet.
If the "H" calculated is positive, the pump can operate
at a suction lift of maximum "H" feet of head.
If the "H" calculated is negative, an inlet pressure of
minimum "H" feet of head is required.
H f
P b
Fig. 43 Minimum inlet pressure - NPSHR
MTR(E), MTC, MTA
tm
(°F)
Hv
(Ft)
370 413
360 328
340 259
320 203
250 66
230 49
39
212 33
194
26
20
176 16
13
158 10
140 6.6
122
4.9
3.3
104 2.6
2.0
86 1.3
0.9
68
0.7
50
32
0.3
Note: In order to avoid cavitation, never select a pump
whose duty point is too far to the right on the NPSHR
curve.
Always check the NPSHR value of the pump at the
highest possible flow rate.
Q
H
H v
300
280
270
148
131
115
98
82
TM03 4307 2006