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 (2)

Pitot Pump

In the previous post, low-flow high head application was introduced as one of the situations to choose reciprocating pumps. There was also a discussion on the low-flow high-head situations when selecting a high speed integrally geared centrifugal pump is preferred. Here another option for such an application is addressed.

Pitot pumps are considered to be a competitive option by many engineers in industry. Pitot pump is a specially designed centrifugal pump which extends the specific speeds of centrifugal pumps down to 50 o 350 (US units), namely low-flow high-head applications. Single stage pitot pumps with capability of delivering flows up to 800 gpm with a head up to 5500 ft have been successfully utilized.

1. Pitot Pump Design

An example of a pitot pump is shown in below figure. It comprises a closed rotating casing with a stationary pitot tube that extends into the rotating case along the axis of rotation with its inlet positioned near the maximum inner diameter of the casing. The liquid enters casing along the axis of rotation and picks up momentum as it passes through the enclosed radial vanes of the impeller into the rotating casing. The liquid is spun up to approximately the full rotational speed of the casing. It then impacts the inlet orifice of the pitot tube near the periphery of the rotating casing (where the pressure and rotational velocity of the liquid mass are the greatest). The liquid is discharged through the inner passageway of the pitot tube and out of the pump.

Pitot pump [Source: Roto-Jet High Pressure Pitot Pumps, Weir Specialty Pumps, www.weirsp.com]

2. Pitot Pump Performance

The head developed by a pitot pump is the sum of two components; the static pressure head created by centrifugal force and the velocity head. This sum will be approximately 1.6 times the head produced from a conventional centrifugal pump of the same size and speed.

Total developed head by a pitot pump can be calculated from the below equation:

where r is centerline radius of the pitot tubr inlet (in) and N is rotational speed (rpm). This equation is not significantly different from the theoretical head produced by a conventional centrifugal pump. It is the combination of more effective conversion of centrifugal and velocity heads developed by the pitot pump and minimal friction that allows this type of pump to develop high heads at relatively modest speeds and at good overall efficiencies.

There are two methods of changing the performance characteristics of the pitot pump; changing the size of the pitot tube and changing the speed at which the pump is operated. In order to represent both of these methods, a typical performance curve is generated for each pitot tube size with a series of head and power curves for each of the common operating speeds (see below figure).

Pitot pump typical performance curve [Source: Angle, Roudnev, Application of the Pitot Pump, Tutorial on Special Purpose Pump, Proceedings of the 14th International Pump Users Symposium, pp144-149]

Pitot pump is suitable for operation over a wide flow range. It can be safely operated at any point on its curve from full flow to shutoff.  It can be operated at its minimum flow indefinitely without any damage.

The head produced by the pitot pump can be easily adjusted by changing the speed at which the pump is operated. Pitot pumps are commonly driven either by electric motors controlled by a VFD, through a gear box used as a speed increaser or by V-belts and sheaves.

Reference: Angle, Roudnev, Application of the Pitot Pump, Tutorial on Special Purpose Pump, Proceedings of the 14th International Pump Users Symposium, pp144-149