The Life Cycle Cost Advantages Of The Triplex PD Pump

The Life Cycle Cost Advantages Of The Triplex PD Pump

The Life Cycle Cost Advantages Of The Triplex PD Pump

The global pump market is dominated by the roto-dynamic principle, mainly centrifugal pump, so it is not unusual for system designers and equipment specifiers to take this as their default choice when considering pumps for specific applications. However, roto-dynamic pumps have limitations in respect of the pressure they can generate and their overall operating efficiency, factors that can influence life cycle costs. It is issues such as these that make the Triplex Positive Displacement (PD) pump an attractive alternative for high pressure duties.

There can be no doubt that if the most appropriate type of pump is selected for a specific application and duty, it follows that  there will be a benefit to the overall performance and life expectancy of the pump. Whereas at one time it was not unusual to find OEMs selecting pumps on the basis of the cheapest pump that would work, today there is far more concern about reliability, efficiency, availability of spares, back-up and energy consumption as their customers are asking for lowest life cycle costs.  For instance, whilst the initial capital investment cost will be higher for a larger reciprocating PD pump than perhaps originally considered, the energy consumption will remain about the same as for a smaller, faster-running pump. The advantage is that long-term operating costs can actually be reduced, as consumption of spare parts will be lower, reliability will be higher and downtime will be lessened, leading to lower life cycle costs.

The triplex PD Pump
A Cat Pumps triplex PD pump employs three synchronised plungers or pistons operating 120° out of phase, which gives it the ability to deliver a predictable and constant flow. This flow is far smoother than that of simplex and duplex pumps. The peak flow in a simplex (single piston, plunger or diaphragm) pump is over three times the mean flow rate and for 50% of each cycle there is no flow at all.  Even a Duplex (twin chamber) pump has a peak-to-mean variation of 157% and twice per cycle the flow drops to zero.  In contrast, for a triplex PD pump (three pumping chambers) in which the pumping stokes overlap, the peak-to-mean variation is only 5%. This is a huge gain compared to the simplex and duplex pumps, and the benefit is vastly reduced flow and pressure variations in the discharge line. The much smoother flow greatly reduces the possibility of system failure caused by excessive pulsation.

Life Cycle Costs (LCC)
One of the essential elements in successfully identifying and selecting the best pump for a particular application has to be the analysis of life cycle costs. The issue of equipment lifetime operating and downtime costs has never been higher within industry, and life cycle costs are now very much a part of the specification process. Putting the purchase price ahead of performance is a common scenario, so the impetus is on the pump provider to examine and explain life-cycle costs and how they can impact on the investment to the customer. Life-cycle costs (LCC) will take into account known factors such as the initial investment and subsequent spares, alongside the less obvious factors associated with maintenance, downtime effect, power consumption and efficient use of the product being handled.

When considering competing high pressure pumps LCC analysis determines the most cost-effective option to purchase, operate, maintain and finally dispose. This is assuming that each pump type is equally appropriate to be implemented on technical grounds. It is worthwhile understanding in detail how the lowest life cycle costs have been achieved for Cat Pumps.  If we look at each stage of the life cycle process we can see how each component affects life cycle costs.

Operating costs
The initial cost of purchase is the smallest contributor to any LCC analysis, with estimates varying from 5% to 15% of total life cycle costs. Correct sizing of the pump to the application will ensure that the purchase cost is kept to a minimum. Then there are the costs of operation, these probably being the largest contributors coming in at between 60% and 80% to the total. The key factor for the pump’s energy consumption is to know the pump’s mechanical efficiency; in other words, how efficiently energy input for driving the motor is converted into output of liquid flow and pressure.
The pump Mechanical Efficiency (ME) is calculated as;

Differential Pressure (bar) x Capacity (m3/hr) / power (kW) x36 = ME x100
Therefore for a Cat Pumps model 3520 plunger pump, with Flow = 87 l/min or 5.2 m3/hr, Pressure = 140 bar and Power Requirement = 24 kW the ME calculation is: 140 x 5.22/23.56 x36 = 730.8/848.31 = 0.86 = 86%.

Therefore, the higher the efficiency the lower the energy costs will be to deliver the required pump output. This becomes a very significant factor when pumps are run for long durations or in some cases continuously. Cat Pumps are very efficient, operating generally at above 85% efficiency compared to other high pressure pump types such as centrifugal, Pitot tube and diaphragm pumps all of which have a much lower efficiency rating.

Maintenance costs

The cost of maintenance is usually the second highest cost affecting LCC and can vary between 15% and 30%, (depending upon pump use and service intervals). Triplex plunger pumps are well known for their high durability, long life and low maintenance. Instances of pumps still in operation after more than 20 years are not uncommon and, subject to recommended routine maintenance, should go on for another 20.
Pump service intervals will depend upon a number of factors, including:
•    Duration of pump operation in hours per day/week/year.
•    Cleanliness of fluid being pumped – Cat Pumps require filtration of the fluid to 50 microns prior to the pump inlet.
•    Temperature of the fluid – Cat Pumps can handle inlet temperatures up to 71°C.    
Temperatures above this level can be accommodated with special seals.

For typical applications Cat Pumps recommend that service is carried out at the following intervals;
Oil change every 500 hrs continuous operation.
Seal change every 1500 hrs continuous operation.
Valve change every 3000 hrs continuous operation.

By way of illustration, a Cat Pump model 3520 operating 5 hours a day, 5 days a week for 40 weeks a year, the service periods would become:
Oil Change: every 6 months
Seal Change: every 18 months
Valve Change: every 3 years
In this typical application, service costs are less than £500 per year over a three year period

Cost of Downtime
The cost of pump downtime is another expense that adds to LCC. Typical examples are unexpected breakdowns, poor performance and disruption to the equipment in which the pump is operating. These factors will impact on plant performance and can lead to significant disruption and additional costs. Therefore pump reliability will be significant in ensuring life cycle costs are achieved. 

Quality build
The build quality of the pump is fundamental to the long term operation of a pump and its suitability for the application. Components that Cat Pumps designers have given particular attention to include manifolds, valves, ceramic plungers, seals, crankcase, crankshaft and connecting plunger rods.

It is essential that an analysis of a pump’s life cycle cost should be part of any selection process. The focus needs to be on the ongoing costs of ownership rather than the initial purchase cost, assuming that the pump is ‘right sized’ for the application. The largest contributor to the cost of ownership is energy consumption, so the efficiency of the pump must be ascertained in order to determine what the energy costs are likely to be over a set period. Maintenance is the next significant component of the LCC, so the quality of the pump build has to be of the highest levels of integrity in order to ensure complete functionality at all times and extended working life. 


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