Opportunities to reduce costs

When faced with replacing an ageing pump, engineers often install a like-for-like replacement, even though this can mean missing out on more energy efficient technology. Concerns often relate to the cost and disruption of selecting alternative equipment. The scale of these problems is, however, frequently over-estimated.

The replacement or refurbishment solution can be chosen according to the budget available. With today’s cost analysis technology it’s very easy to calculate the payback period, and it then becomes a simple trade-off, to balance the outlay with the potential cost and carbon reductions.

Many improvements can be made leaving the plant room pipework untouched. Just keep the existing body casing with inlet/outlet connections in place, and remove the rest of the pump set. Then replace it with a new close coupled pump, supplied without its body casing. This rotating assembly can be fitted with an integrated frequency inverter if required to bring the benefits of variable speed drive operation.

Low-hanging fruit

So where do you look first for inexpensive but extremely beneficial improvements?

If you can answer ‘yes’ to any of the following then there are financial and energy efficiency benefits to be had.

  • Are any of the pumps belt-driven?
  • Are they started by direct on line or Star Delta starters?
  • Are cold water booster sets fitted with horizontal pumps?
  • Are commissioning valves on the main headers part closed?
  • Is the heating or chilled water system fitted with three port valves and by-pass circuits?

Each of the following options assumes that the equipment to be replaced is a belt driven pump with duty of 20l/s at 160kPa. The cost savings are calculated on the basis of an electricity cost of 10p/kWH and 8000 hours/year running.

Option 1 – the opportunity

Most of the old belt-driven pumps installed have been oversized, to allow for possible extra system resistance or future duty increases. The pump end operating speed was only ever a near fit to the actual duty because the limited number of pulley sizes available reduced the number of speeds available for selection. Most would end up over-sized if they were not throttled back on the main commissioning set. So they’re wasting power over a flow/head point in excess of the real need.

The belt and pulley drive absorbs power, reducing pump operating efficiency and wasting money. If the pulley drive is not in A1 alignment and tension, it will become noisy, potentially leading to issues with the building occupants. And when it’s in this condition, the drive absorbs even more power, increasing the relative cost of ownership.

Option 1 – the solution

Replace the belt-driven pump with a close coupled relative of the installed pump, using the existing pump casing and pipework. The impeller in the replacement pump is trimmed to the exact duty requirements, allowing the commissioning set to be opened-up, reducing power absorbed and saving money. There are no energy-wasting belt drive losses. The motor is also sized to the actual working duty, not over-sized, so it is more energy efficient and saves money.

This replacement option also brings quieter operation and reduced maintenance requirements. Belt-driven pumps need their belt tension and pulley alignment checked and adjusted, which is not necessary on close coupled pumps.

Option 1 – installation

No pipe changes are needed as the new rotating assembly fits into the old pump casing and uses the same input/output connections. The close coupled pump will be approximately 150mm longer, so the designer needs to check that there is sufficient space behind the pump before installation.

Option 1 – savings

This is the lowest capital outlay option, but savings would still be £864 per annum. For example, an 80-250 Starflex belt drive pump absorbs 5.77kW at the projection of the duty point to the 1450 rpm curve. An 80-250H Starbloc close coupled replacement pump absorbs 4.69kW with impeller trimmed to the exact duty, minimising waste energy. This would deliver a 17% saving in absorbed power.

Option 2 – the opportunity

Most old pumps are running at 100% design flow all the time with the heating or cooling controls by-passing fluid through a parallel circuit, using a three port control valve, when it is not needed.

Variable speed drive pumps offer an excellent replacement, but it is often thought that this will be complex and involve large-scale work. This is not the case. The move to variable speed can be made without altering pipework, and without the need for BMS or differential pressure sensors.

Option 2 – the solution

Replace constant water volume, fixed speed, belt-driven or direct driven pumps on three port control, with variable speed, variable volume pumps with two port operation. In addition to giving all the advantages of option one, this reduces power consumption at part load, for significantly reduced running costs. Typically, most heating and cooling systems operate at 50% design flow. The pump must slow down by about 15% to match its output to this reduced demand. Because centrifugal pumps have a cube law relation between speed and absorbed power, this can reduce power absorbed by 39%.

Option 2 – installation

No pipe changes are needed. Keep the existing body casing with the inlet-outlet connections in place but remove the rest of the pump set. Replace it with a close coupled pump with integrated frequency inverter, converting from three port control valves to two port operation. The by-passes become redundant. With sensorless software built into the drive, the motor will automatically sense when the two port valves are opening or closing and adjust the speed, and hence the output, of the pump to suit.

Option 2 – savings

This option delivers savings of £3,400 per annum at continuous 50% load. For example, an 80-250 Starflex belt drive pump absorbs 5.77kW at the projection of the duty point to the 1450 rpm curve. An 80-250H Starbloc close coupled variable speed IVS Sensorless pump absorbs 1.5kW at 50% design flow, running at 1000 rpm. A 30% reduction in speed produces an energy saving of 74%, compared with fixed speed.

Option 3 – the opportunity

If the budget allows, the greatest potential cost saving per annum would involve changing out fixed speed, belt-driven or close coupled pumps completely and installing a new high efficiency unit benefiting from sensorless variable speed drives.

Option 3 – the solution

There are a wide range of low carbon opportunities, including the latest control technology, which balances demand across the system to achieve the optimal wire to water efficiency based on the most efficient use of each component. Pipeline mounted vertical in-line pumps can be considered, which remove the need for inertia bases, and there are numerous packaged/offsite manufactured options, such as Armstrong’s Pumpbox.

Option 3 – the savings

To give one relatively simple example, an 80-25- Starflex belt drive pump absorbs 5.77kW at the projection of the duty point to the 1450 rpm curve. An 80-250A Armstrong 4280 close coupled variable speed IVS Sensorless pump absorbs 1.07kW at 50% load, speed 1010 rpm. A 30% reduction in speed produces a 90% plus reduction in absorbed power compared to the original belt driven pump. This equates to annual cost savings of £3750 at continuous 50% load.

The payback calculator can be used to analyse figures for other pump sizes and before/after combinations.

In summary, the choice of outlay and level of work can be selected to suit the project budget, but the opportunities for cost saving
and carbon reduction are widespread and easier to achieve than many might think.

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