Steam: Not a free ride
29 Jan 2007
Steam is widely used as an energy-transport medium, distributing heat and power in many industrial applications. The amount of money spent generating steam is very large: equivalent to about 40% of the fuel burned by the process industry.
The extensive use of steam means that effective use of flowmeters can lead to significant cost savings in most steam distribution systems. However, flowmeters are not always installed in steam systems and many meters are not used to best advantage, in part because steam has traditionally been perceived as a "free" resource in the process sector.
Estimates show that the cost of generating steam ranges between 50p and £25 per tonne - depending on the fuel used and whether it is being generated for a specific purpose or is a by-product of some other process (see table 1). But as the value of steam is not always appreciated, uptake of good metering practice has been slow in some sectors. Legislation and taxation measures such as the Integrated Pollution Prevention and Control (IPPC) and the Climate Change Levy (CCL) seem to have have helped initially, but this effect has not been sustained.
The drive towards improving process efficiency has, however, resulted in more interest in flow metering in the process sector, as has the trend towards purchase of steam from third-party operators of CHP or boiler plant.
Market pressures continue to drive technological developments with a slow but steady improvement in the design of existing meters and very rapid development, also, in newer metering methods.
Industrial steam systems operate with either saturated or superheated steam. The requirements and behaviour of these systems differ, which affects flowmeter selection.
Most process and heating steam systems use saturated steam and it is important to note that some condensate (liquid water) is always present in saturated steam pipework. Steam quality — the mass fraction of steam vapour in the steam-condensate mixture — is usually better than 0.9, though poorly maintained systems may have large amounts of condensate present.
Steam density can be determined with a single measurement of either temperature or pressure. If heat is lost from saturated steam, more steam will be converted to condensate and the steam quality will reduce, but its temperature will not change at set operating pressure.
Saturated steam can be superheated by adding heat or by a sudden drop in pressure, while flow velocity should usually limited to about 35ms-1 to prevent condensate erosion problems. It is often used in main systems that feed multiple points of use and variable demand, typically, causes large fluctuations in flow rate and pressure in these systems
Superheated steam is mostly used in power generation applications and on large sites supplied by CHP plant. Condensate is not usually present in superheated systems (i.e. the steam quality is 1). However, condensate will be present during start-up and the system should be designed and operated to account for this.
If heat is lost from superheated steam, its temperature will reduce at set operating pressure and if sufficient heat is lost it will become saturated. Superheated steam systems typically operate at 50-70ms-1, and to determine steam density steam temperature and pressure must be measured.
Another factor is steam enthalpy — a measure of the energy contained in the steam, which varies significantly with temperature, pressure and, for saturated steam, the steam quality.
A problem that can occur in any steam system is "waterhammer", particularly at start-up, when slugs of condensate are blown through the pipes. This can cause major damage to pipework and flowmeters. Consequently, properly designed steam systems (both saturated and superheated) should include condensate traps and separators.
Ideally a flowmeter will be accurate, reliable, robust and easy to use and install. However, no one type of meter is ideal for all steam applications and most designs have advantages over each other under certain circumstances (see table 2). Obviously the flowmeter must be able to operate at the steam flow conditions or else methods must be available to insulate it from the steam. The inherent variability of flow and pressure in steam systems makes a high turndown ratio important in many applications.
Nearly all steam metering applications require a mass flow measurement. This can be done either by direct output of mass flow or a volumetric output combined with a steam density measurement.
Insensitivity of flow measurements to wetness is particularly important in saturated steam flow measurement. It is more difficult to perform an accurate calibration of a flowmeter in steam than in most other fluids. Indeed, any metering method that eliminates the need for calibration offers advantages over flowmeters that require calibration at operating conditions.
In general, steam pipework is over-sized the flowmeter design should allow the use of restricted measuring sections or whose measuring range can be re-ranged to accommodate this.
The above issues are well recognised by meter vendors, who generally provide good advice to their customers. However, it might not always be commercially sensible for vendors to emphasise the difficulties of metering steam so it can be better to seek independent advice from consultants, trade bodies and industry groups.
The extensive use of steam means that effective use of flowmeters can lead to significant cost savings in most steam distribution systems. However, flowmeters are not always installed in steam systems and many meters are not used to best advantage, in part because steam has traditionally been perceived as a "free" resource in the process sector.
Estimates show that the cost of generating steam ranges between 50p and £25 per tonne - depending on the fuel used and whether it is being generated for a specific purpose or is a by-product of some other process (see table 1). But as the value of steam is not always appreciated, uptake of good metering practice has been slow in some sectors. Legislation and taxation measures such as the Integrated Pollution Prevention and Control (IPPC) and the Climate Change Levy (CCL) seem to have have helped initially, but this effect has not been sustained.
The drive towards improving process efficiency has, however, resulted in more interest in flow metering in the process sector, as has the trend towards purchase of steam from third-party operators of CHP or boiler plant.
Market pressures continue to drive technological developments with a slow but steady improvement in the design of existing meters and very rapid development, also, in newer metering methods.
Industrial steam systems operate with either saturated or superheated steam. The requirements and behaviour of these systems differ, which affects flowmeter selection.
Most process and heating steam systems use saturated steam and it is important to note that some condensate (liquid water) is always present in saturated steam pipework. Steam quality — the mass fraction of steam vapour in the steam-condensate mixture — is usually better than 0.9, though poorly maintained systems may have large amounts of condensate present.
Steam density can be determined with a single measurement of either temperature or pressure. If heat is lost from saturated steam, more steam will be converted to condensate and the steam quality will reduce, but its temperature will not change at set operating pressure.
Saturated steam can be superheated by adding heat or by a sudden drop in pressure, while flow velocity should usually limited to about 35ms-1 to prevent condensate erosion problems. It is often used in main systems that feed multiple points of use and variable demand, typically, causes large fluctuations in flow rate and pressure in these systems
Superheated steam is mostly used in power generation applications and on large sites supplied by CHP plant. Condensate is not usually present in superheated systems (i.e. the steam quality is 1). However, condensate will be present during start-up and the system should be designed and operated to account for this.
If heat is lost from superheated steam, its temperature will reduce at set operating pressure and if sufficient heat is lost it will become saturated. Superheated steam systems typically operate at 50-70ms-1, and to determine steam density steam temperature and pressure must be measured.
Another factor is steam enthalpy — a measure of the energy contained in the steam, which varies significantly with temperature, pressure and, for saturated steam, the steam quality.
A problem that can occur in any steam system is "waterhammer", particularly at start-up, when slugs of condensate are blown through the pipes. This can cause major damage to pipework and flowmeters. Consequently, properly designed steam systems (both saturated and superheated) should include condensate traps and separators.
Ideally a flowmeter will be accurate, reliable, robust and easy to use and install. However, no one type of meter is ideal for all steam applications and most designs have advantages over each other under certain circumstances (see table 2). Obviously the flowmeter must be able to operate at the steam flow conditions or else methods must be available to insulate it from the steam. The inherent variability of flow and pressure in steam systems makes a high turndown ratio important in many applications.
Nearly all steam metering applications require a mass flow measurement. This can be done either by direct output of mass flow or a volumetric output combined with a steam density measurement.
Insensitivity of flow measurements to wetness is particularly important in saturated steam flow measurement. It is more difficult to perform an accurate calibration of a flowmeter in steam than in most other fluids. Indeed, any metering method that eliminates the need for calibration offers advantages over flowmeters that require calibration at operating conditions.
In general, steam pipework is over-sized the flowmeter design should allow the use of restricted measuring sections or whose measuring range can be re-ranged to accommodate this.
The above issues are well recognised by meter vendors, who generally provide good advice to their customers. However, it might not always be commercially sensible for vendors to emphasise the difficulties of metering steam so it can be better to seek independent advice from consultants, trade bodies and industry groups.
