Pump and Motor Failure in a Hot Potassium-Carbonate System

A case-history study of a pump failure and the corrective measures introducedto prevent future breakdowns.

Dennis A. Novacek, Farmland Industries, Inc., Enid, Okla. 73701Farmland Industries, Inc., Enid, Oklahoma fertilizer facil-ity includes two 1000 TPD rated ammonia units of M. W.Kellogg design. Both units utilize a hot potassium-carbonate system for carbon-dioxide removal from the pro-ces s gas.

These systems are a standard design consisting of twolevels of regenerated solution circulation from the regen-erator tower; lean and semi-lean carbonate. Unlike manyunits, however, none of the solution-pump drivers aresteam turbines. Lean-solution circulation is maintainedbyspared electric driven pumps. Similarly, semi-lean circu-lation is normally-provided by one of two motor-drivenpumps, supplemented by a pump driven by a hydraulicturbine. As a result, system circulation, carbon-dioxide re-moval, and ammonia production are wholly dependentupon the availability of electric power.

To provide a dependable source of electricity, the plantis supplied by two separate full-power lines from the localutility, with a tie-breaker closure for situations of voltageloss on either of these lines. Critical electric-drive equip-ment, including the solution pumps, is rejayed to start au-tomatically.

In Oklahoma, spring is a time for many, often violent,thunderstorms. During these periods, the electric utility issubject to numerous power dips or "flickers", and an occa-sional complete loss of power on specific Unes. The Enidplant normally maintains power and operation throughone of its feeder lines, the tie breaker, and automatic motorrestarts. Because the carbon-dioxide solution pumps areall electric driven and must be operating to keep the am-monia unit in production, operators are naturally sensitiveto bad weather and electric dips or loss.

Figure 1. Semi-lean carbonate pumps.

Plant/Operations Progress (Vol. 2, No. 4)

In May, 1980, during a thunderstorm, a momentarypower dip dropped the semi-lean solution pump off-line.The motor did not restart automatically as it normallyshould, and also did not start from the control-board startstation. Two operators immediately went to the pump inan effort to start it locally, and avoid abandoning the con-verter loop and methanator. The motor was energized atthe local start-stop station, but, after doing so, it was appar-ent that the pump had been rolling backwards, so it wasimmediately shut down. The discharge check valve hadfailed to close, resulting in a reverse flow through thepump from the absorber to the regenerator. The operatorsfirst attempted to stop the flow by manually handjackingthe 10 in. flow-control valve on the discharge shut, butwere unable to do so. They then attempted to close the 12in. discharge block, but abandoned the area as the pumpand motor noise continued increasing and the solutionlines started jumping off the supports. The pump and mo-tor both blew apart within 5 to 10 seconds after the person-nel left the area. Neither operator was injured, since theywere able to run around a small analyzer building about 30feet away. The pump had, in effect, operated as a hydraulicturbine, letting down pressure from the absorber to the re-generator and oversped.

The personnel involved were very fortunate to avoid se-rious injury. Small pieces of the pump and motor werefound 100 to 150 feet away. The entire shaft blew out of the1000-HP motor and came to rest about 35 feet from thestarting point. The marks on the pavement in the area indi-cate a 60-foot spinning route deflected at two points byconcrete foundations. The motor was wrecked beyond re-pair, since both bearing caps broke off'and the shaftsheared at the step before the coupling, and then wentthrough the outboard end of the motor. The rotor lamina-tions came loose from the shaft and stacked up in the airducts. On the pump end, the coupling pulled off the shaft,the bearing housings tore loose from the case, and the sealglands came loose. Obviously, the case and impeller took asevere beating. The impeller actually welded itself to theupper half of the case.

It is unknown how fast the pump and motor were reverserotating, but the hydraulic turbine driving an identicalpump overspeed trips at 4250 rpm compared to a normaloperating speed of 3550 rpm. The pump must have beenrolling at or in excess of this speed.

Inspection of the check valve revealed only that an anti-rotation pin holding the disc had broken and the disc andseat ring had worn. Given the destruction of the equip-ment, it seems likely that the check had hung full open.These check valves had been troublesome in the past andseveral modifications were made, both by Farmland andthe manufacturer, including anti-rotation pins to hold the

October, 1983 209

Figure 2. Figure 3.

Pump and motor immediately following the failure.

Figure 4. Close-up of the pump coupling end. Figure 6. Final location of the motor shaft over 35 feet from the motor.

Figure 5. Upper half of the pump case as removed.

disc and stops to limit travel in the open position. Previousincidents had caused some damage to pumps and motors,but nothing approaching the magnitude of this failure. Inthese cases, either the pump was successfully isolated orthe check valve eventually had reseated itself. Because ofthis history, inspection of the checks were routine duringmaintenance turnarounds. With such a serious near-missfrom another check valve failure as this one, it was deter-

210 October, 1983

mined to find another means of checking backflowthrough the pumps.

The solution used is simple and inexpensive. Eachelectric-driven pump is throttled by a separate controlvalve controlling total semi-lean flow. These valvesworked in parallel from the flow signal of the total semi-lean solution header which includes flow, if any, from thehydraulic turbine-driven pump. All flow adjustmentswere taken on these control valves on the electric-pumpdischarges, and both valves were opened in tandem,irrespective of whether the specific pump was in opera-tion. The fix was to add a three-way solenoid valve on thecontrol air to the_valve actuator. This solenoid valve allowsthe control valve to open only if the motor and pump asso-ciated with it is operating. This applies equally whetherthe pump is purposely down or trips-off as a result of apower loss. Only if the motor is operating can the valve beopened. In this way, the pump is isolated on the dischargeside by the control valve if not in operation, and is stillready for immediate startup when needed. This arrange-ment had worked without exception on numerous powerdips or failures since. In fact, the control valves open andclose rapidly enough so that the pumps can be switchednearly bumplessly from the control room start-stopstations.

In these days of high technology, it is often failure of rou-tine, ordinary equipment that causes the most serious acci-dents. In a modern chemical-process unit, there are few

Piant/Operations Progress (Vol. 2, No. 4)

things as simple as a standard swing check valve. Eventhough the consequences of this accident were not reallyvery severe, we feel that the potential seriousness to per-sonnel was such that an improvement to the operatingeharacteristics of these valves was mandatory. We recom-mend our solution to anyone with a similar problem.

Dennis A. Novacek is Technical Superintendent atFarmland Industries' Enid, OK, Fertilizer facilityand is responsible for the engineering, laboratory,and regulatory functions. He received a BSCHEwith distinction from the University of Nebraska.

How to Moke Your Meter ing Pump Operatethe Way You Want It To

Metering pumps are only too often poorly suited to the task for which theywere purchased. A down-to-earth discussion of how to avoid this situation.

Raymond L. Jewett, Liquid Metronics, Inc., Acton, Mass. 01720

The pumps I shall talk with you about are not the pumpswhich serve the petrochemical industry as the motiveforce for the mainstream of production. They are not thepumps that produce pressures of 30,000 psi or more norflows of hundreds of gallons per minute.

The pumps that are my subject (Figure 1) serve thepetrochemical industry in auxiliary functions, the mostcommon of which are the several forms of problem-watercorrection that are carried on in the industry:

1. Cooling tower treatments2. Boiler water treatments

Figure l. Tank-mounted dosing (metering) pump. Electronic control type.

Plant/Operations Progress (Vol. 2, No. 4)

3. Waste flow treatments4. Potable water treatments5. Miscellaneous process applications—e.g.:

a. Corrosion inhibitor injectionb. Emulsion breaker feedc. pH control chemical feeds

SPECIFIC REQUIREMENTS

There is no more important facet of the sale and use ofsuch pumps than the requirement that the pumps fit theinstallation to which they are to be applied, and yet wecontinue to see specifications for metering pumps whichare poorly suited for the task for which they are purchased.In an uncomfortable percentage of the cases where, for onereason or another, we compare the specific requirementsof the application with the specifications on the bid re-quest, we have to conclude that the specifier either didnot know his application well, or that hè had a good friendin the pump business. The specifications too often fit thepump available for sale, rather than the job to be done.

These specifying errors usually result in oversizing. It isunfortunate to see an operator trying to cope with the prob-lem of operating a motor-driven metering pump at an out-put of 2% to 5% of its maximum output, but such cases aremore common than you might think. We seem to apply tometering pumps the assumption that bigger is alwaysbetter.

It is as unfair to expect a 200 gallon-per-day meteringpump to perform well at a rate of 2 gallons per day as is theopposite expectation. It just is not as obvious.

Remember that the time you save in not carefullydetermining the requirements of a chemical feed systemwith respect to output volume and pressure is a one-timesaving that is bought at the cost of long-term operatingproblems and costs. Metering pumps are temperamentalbeasts, even when they are well fitted to their tasks. Theydon't need to be handicapped by ill-suited application.

For instance, the operations of a motor-driven meteringpump's drive mechanism (Figure 2) and valves at one-tenth of the pump's maximum speed simply are not thesame as they are at maximum speed. To assume that theyare is to invite continual problems for the unfortunateoperator.

October, 1983 211

jat. '"aiFigure 2. Drive mechanism of heavy-dury motor-driven metering pump

showing hearing points and multiple linkages.

Motor Speed and Flow Velocity

The pump's motor speed affects the flow velocitiesthrough the valves and other liquid-end components, andthe velocity of the flow through the valve elements is animportant determinant of their performance (Figure 3). II-lustrate this mentally by picturing your attempt to inflatean automobile tire with a hand pump, preferably one thatis well worn. If you operate the handle up and down withsufficient vigor and velocity, you will move air out of thepump and into the tire. If, on the other hand, you let theplunger of the air pump descend into its cylinder undernothing more than its own weight, air will ooze by theplunger, and nothing will enter the tire.

212 October, 1983

Figure 3. Schematic illustration of change in valve-ball lift with change inflow rate.

This is a maximal illustration of a velocity effect that, inmore modest proportions, can afflict any metering pumpwhose change in output is based on varying the speed of itsdrive motor.

Reductions in output achieved by reducing the length ofstroke of a metering pump are plagued by analogous pit-falls. The elasticity of components of both the drive assem-bly, the seals and the liquid end, for instance, will assumegreater importance as stroke length is reduced and willtend to reduce the relative volumetric efficiency of thepump when its output rate must be set in the lowestportion of its range.

But to point out these problems does not teil us how tomake sure that the metering pump we choose for a givenfunction really does fit the requirements of the task it is toperform rather than the requirements of the producer ofthe pump.

Determinotion of Total Lood

It is simple, even simplistic, to say that the first thing weshould determine is the total load the pump must carry tomeet the requirements of the installation. That is, againstwhat pressure must it inject, and what total volume offluid must it deliver per unit time. I hope it will come as aconsiderable surprise to you that manufacturers and pumpdistributors receive inquiries and even orders with neitherof those parameters specified.

There are, in fact, instances when it may be impractica-ble to determine both of these parameters very precisely. Ican only remind you that, unless you can specify both,there is every possibility that you will incur the cost ofdetermining them accurately in the form of a misappliedpump and its consequent headaches and expenses.

I remind you again that the time and money you spend inmaking sure you have correct figures for pressure andflow-rate requirements are a one-time cost, whereas an er-ror in specifying may result in a day-after-day expenditureof money you didn't need to spend, continual problemswith feed accuracy, and constant operator frustration. Youneed hear only o nee the caustic comment of a meteringpump operator describing the competence of a specifyingengineer to know that operator frustration can be asignificant problem.

Knowing with some exactitude the requirements of thetask for pressure and output is a great leap forward, but it isnot always a home run.

System Configuration

Metering pumps are typically installed in one of two ma-jor configurations: 1) constant-rate, (Figure 4) or 2)Instrument-responsive (Figure 5). We need also to deter-mine which of these configurations is appropriate for ourtask.

Plant/Operations Progress (Vol. 2, No. 4)