CHPQA Scheme Benefits - Good Instrumentation Key

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CHP plants that meet the requirements of the CHPQA scheme qualify for a number of valuable benefits. Alan Hunt, Product Specialist for ABB Measurement & Analytics in the UK, explains how measuring CHP plant performance through good instrumentation is the key to gaining them.

*Alan Hunt, Product Specialist for ABB Measurement & Analytics*

In today’s energy climate, it’s easy to see why Combined Heat and Power (CHP) plants are so attractive. Essentially, CHP plants generate electricity while capturing the heat produced during the process for other uses, such as industrial production or district heating schemes. Compare this with conventional electricity production where vast amounts of heat are simply released to the atmosphere via cooling towers, wasting up to two thirds of the energy consumed.

By making use of this otherwise wasted heat, CHP plants can be 80 percent efficient, compared to the figure of around 50 percent for most UK gas fired power stations.

This huge gain in efficiency means it’s no surprise that the government is trying to encourage the installation and operation of good quality CHP projects by offering various benefits.

It does this through the Combined Heat and Power Quality Assurance (CHPQA) scheme, a voluntary programme that assesses the energy and environmental performance of CHP projects. Those operators that can show that their plant is eligible can currently benefit from the Climate Change Levy Exemption, Enhanced Capital Allowances and exemption from business rating for CHP plant and machinery.

Proving eligibility for the CHPQA scheme

To make their plants eligible for Climate Change Levy exemption on the gas used to fuel CHP schemes, operators must achieve certain metering criteria. These can be divided into two scheme types – those below 2MWe and those above 2MWe.

Above 2 MWe

For these schemes, operators need to be able to measure the gas used, the heat supplied and the electricity generated. The gas meters must be pressure and temperature compensated and the heat meters must use a water meter to measure actual flow.

Below 2 MWe

Schemes below 2 MWe can be further subdivided into those that have heat dumps and those that do not:

CHP Schemes less than 2MWe WITH Heat Dumps

These schemes require various combinations of gas, electricity and heat metering. If the unit is less than 500kWe and is on the approved CHP list as issued by the CHPQA, then the gas consumption may currently be calculated by using the CHP units’ electrical efficiency as stated on the list.

However, if the unit is greater than 500 kWe or below 500kWe but not on the approved CHP list, then gas metering is required and the meter must be pressure and temperature compensated.

The gas meter installed must be of a quality specified by S.I. 1983/684 (Statutory Instruments 1983 No. 684, Gas (Meters) Regulations 1983).

For metering electricity, clearly labelled commercial/industrial three-phase electricity meters of billing quality should be used.

CHP units will normally supply heat as either hot water or steam. For measurement of steam mass flow and energy content, meters with an overall uncertainty of ±2.0% of span (full-scale) are required.

For measurement of hot water, where stand-alone commercially available heat meters are used these must be manufactured to metrological Class 2 or Class 3 as defined in BS EN 1414–1997 (~ ±1.5%). For fluid temperature measurements, matched pairs of platinum resistance thermometers to BS 1041-3:1989 and BS EN 60751:1996/IEC 60751:1983 are preferred.

CHP Schemes less than 2MWe WITHOUT Heat Dumps

For these schemes, the measurement of gas and electricity is the same as for units that have heat dumps installed. The difference comes in the measurement of heat.

Existing schemes below 2MWe with no heat rejection facility are not required to meter heat outputs, as the heat output may be calculated using the design Heat/Power ratio for the unit. For new schemes or those which supply heat in the form of steam, heat must be metered in the same way as units greater than 2MWe.

Oil burned as either a main or a standby fuel will need to be monitored and recorded. Oil products are usually sold by the litre and this will require a volume flow meter with an uncertainty no greater than ±1%.

CHPQA is an annual certification process. At the end of each calendar year, a designated Responsible Person must compile the CHP scheme energy data monitored over the previous year; assess the scheme’s performance and submit this data to CHPQA on the appropriate forms for validation.

Good boiler operation is key

Operating the boiler in the optimum way is key to an efficient CHP plant and measurement is the way to ensure this. The key is to make sure you are only generating what you need.

Measure the demand and compare it with what the boiler is generating to identify whether steam is being lost before it reaches the point of use. This will ensure that the full supply of steam is reaching the place where it’s needed most rather than inadvertently heating up your factory.

It’s also important to optimise the combustion process by monitoring the flue gases. Careful monitoring will allow you to strike a balance between supplying too much air, which carries heat away up the flue, and insufficient air, resulting in incomplete combustion.

It is also important to ensure that boiler duty is at optimum efficiency. For example, don’t use two boilers at 30 percent output if you can run one at 60 – 70 percent output.

Checking that your instrumentation is up to scratch should also be a priority. Modern instruments are typically more robust and more accurate. They are also easier to maintain, and are less prone to problems such as drift.

The challenge of heat

The measurement of electricity and gas flow is relatively straight forward. However, because it depends on the flow of water and in particular steam, the major challenge in CHP schemes is the accurate measurement of heat.

In any boiler based system, the critical measure of the amount of energy flowing round the system is the mass of the steam. It takes ten times the energy to create 1 m3 of steam at 10 bar than at 1 bar, yet the volume is the same.

This has traditionally been the domain of differential pressure meters such as orifice plates. The disadvantage with these is that they require a lot of ancillary equipment such as line pressure transmitters, temperature sensors and a flow computer to produce mass readings for steam. This introduces an extra maintenance burden and thus increases operational costs.

A better alternative is vortex and swirl meters. These have virtually no maintenance requirements and offer a greater level of accuracy, particularly in situations or applications were flow can vary over a large range.

Orifice plates offer an accuracy of only two percent of the upper flow range, whereas vortex and swirl meters can produce an accuracy of 0.5 percent over the entire flow range. Their turndown, or the effective range in which they can accurately measure, is also up to ten times greater than that offered by an orifice plate.

Vortex meters rely on the phenomenon of eddies or vortices which form as an obstruction resists the flow of fluid around it. The meters measure the frequency of the eddies as the fluid passes an obstruction known as a shedder.

By contrast, swirl meters use static veins at the meter’s entrance to induce the fluid to rotate. This automatically sets up a secondary helical rotation, the frequency of which is measured by the flow meter.

The frequencies of the vortex street in a vortex meter and of the secondary rotation in a swirl meter are each directly proportional to the volumetric flow rate of the fluid. There is no need to compensate for changes in pressure, temperature or density – the only parameter the meters need to know to calculate the mass flow is the temperature of the steam.

A major benefit of swirl meters is their ability to be fitted almost anywhere, which makes them ideal for retrofitting to existing steam systems. To achieve the greatest accuracy, the vast majority of flow meters need to receive an undisturbed flow of fluid.

This entails positioning them downstream from any sources of flow turbulence, such as bends in the pipe, valves or other obstructions that could disturb the flow.

For example, vortex meters require straight inlet and outlet runs of up to 50 pipe diameters and 10 pipe diameters respectively following a modulating valve.

Swirl meters need just two or three diameters in most applications, greatly reducing the installation space required. This is possible because the rotating flow pattern inside a swirl meter is specially set up by a series of blades at the inlet to the meter and ‘deswirled’ again at the outlet.

This effectively isolates the flow pattern inside the meter from many of the disturbing influences that can cause inaccurate readings in other designs.

Don’t forget service

Of course, even the most accurate and sophisticated meter will not continue to perform well if it does not receive the correct servicing at the appropriate time. Reputable manufacturers should be able to help with this by offering a service programme tailor made to the instrument and its application.

Other services may be designed to maximise the performance of the instrument. These could include services such as fully-traceable verification of flow transmitters, certifiable inspection of primary flow devices, assessment of the device’s installation, design of optimum calibration schedules for the unit and upgrade services for measurement devices and installation and commissioning of new devices

As well as instrument maintenance, servicing and performance optimisation, some of the more advanced and customer-focused instrumentation vendors may also be able to offer a number of other services.

Some of these will be directly applicable to achieving the benefits available through the CHPQA scheme. Examples could include verification of the F4 self-assessment form; verification services; life cycle services to maintain CHP performance and support for the annual self-assessment process.

The CHPQA scheme is a major encouragement to CHP plant operators – monitoring the performance of such plants by efficient, accurate meters, backed up with the support of innovative vendors such as ABB, will ensure its valuable benefits help make a difference to their business and to the environment.