Pump Performance Monitoring

Pump performance monitoring is a key way to their predictive maintenance (and in turn reducing the maintenance costs associated with preventive maintenance). Briefly, pump performance monitoring means:

1. Monitoring the trend of a pump performance over time;
2. Deciding if the pump needs to be disassembled and repaired.

Pump performance monitoring procedure can be summarized as checking its operating conditions (by measuring flow, suction pressure and discharge pressure and having specific gravity) relative to its performance curve for one or more operating points. The result can be categorized as follows:

(a) Operating point is not on the curve

If the pump's operating point is within 10% of its performance curve, then it is running healthy but if it is operating more than 10% below the curve, it is probably worn internally and needs an overhaul. Refer to the diagram of the below figure for internal wear effects on performance characteristics. Also included in this diagram is state of the pump characteristics for worn (reduced diameter impellers).

Effects of pump internal wear on its performance characteristics

[Source: R. Beebe, "Predictive Maintenance of Pumps Using Condition Monitoring", Elsevier]

 

It is obvious that the flow is reduced as a result of internal wear for a given head; flow through the impellers equals to pump output flow plus leakage flow (which is circulating inside the pump).

(b)  Operating point is on the curve

If the pumps operating point is in the "Equipment Reliability Operating Envelope" (EROP), no action is required. If it is not, pump is likely to suffer from change(s) in process conditions. EROP or "Heart of the Curve" for a pump is typically -50% to +10% in flow of the pump "Best efficiency point" (BEP).

Referring to diagram of below figure, depending on the pump operating point on the curve, different components may have been damaged. Note that in the figure component damages are effects of the possible causes. The important point is that the "Root Cause" for all of these possible causes is "Change in Process Conditions".

Pump component damage and causes as a function of operating points

[Source: W. Forstoffer, "Reliability Optimization through Component condition Monitoring and Root Cause Analysis", Elsevier]

 

Following notes should be considered for pump performance monitoring:

(1) A complete monitoring is ideally performed for the following operating points:

• BEP;
• Some point about 10-25% above BEP;
• Some point about 10-25% below BEP;
• At or near shutoff.

(2) Checking shutoff is important in that a pump with plugged suction line will usually put up the design shutoff head but will operate below the curve at increased flow. So operation of a pump on the curve at shutoff but below the curve at increasing flows can be a sign of suction problems and not pump itself;

(3) Flow can be determined via either:

• Flow meter;
• Motor amps;
• Control Valve position;
• Steam turbine throttle valve position;

(4) In case of no gage existing, suction pressure can be calculated as:
[suction vessel pressure] + [static head (vessel liquid level compared to pump suction centerline)] - [suction piping friction loss];

(5) If speed and impeller diameter of operating pump is different from that of shown on performance curve, "Affinity Laws" shall be implemented to adjust the curve.

A Note on Pump Selection (1)

Selecting Centrifugal or Reciprocating?

Although centrifugal pumps are the first option in a pump selection process because of lower cost, simplicity, reliability and smooth flow characteristics, there are some occasions when positive displacement pumps (reciprocating or rotary) might be more desirable. These occasions can be classified as follows:

1. Very high head, especially when combined with a low capacity; 
2. Low speed drivers;
3. High efficiency requirements over a range of pressures;
4. Constant capacity requirements over a wide range of heads;
5. Variation of head requirements over a wide range;
6. High viscosity;
7. Emulsification;
8. Accurate control of flow requirement in low flow applications.

There are some notes that should be considered together with the above-mentioned selection criteria:

• There is a specially designed type of centrifugal pumps for low-flow, high-head applications. This is high speed integrally geared vertical inline pump which is identified by API classification OH6 and is known as “Sundyne” in industry. As a guideline, reciprocating pumps are selected for low-flow, high-head applications when service conditions are outside the hydraulic coverage range of Sundyne pumps as presented in below Figure. Of course NPSH and minimum flow requirements should be carefully investigated when selecting a Sundyne pump.
• Although initial costs of small reciprocating pumps are competitive with centrifugal pumps, for capacities over 200 gpm they are usually more expensive.
• When service requirements permit using either a centrifugal or a reciprocating pump, the operation and maintenance cost should be carefully considered. Maintenance costs of reciprocating pumps are usually higher because of moving parts, valves and sliding contacts.
• For viscosities above 500 SSU and below 8000 SSU usually reciprocating pumps are considered. For viscosities above 8000 SSU rotary pumps are the better options.

 Sundyne pumps hydraulic coverage