A number of important considerations need to be taken into account when installing desuperheaters. This tutorial covers issues such as water quality and pressure control. A desuperheater selection chart and a list of applications are also included.
There are a number of important considerations to take into account when installing a desuperheater,
A generalised installation of an in-line desuperheater is shown in Figure 15.4.1.
The use of hot water has the following advantages:
There are however, two disadvantages to using high temperature cooling water:
Due to the benefits of using hot water, it is logical to insulate the hot water supply pipes to minimise heat loss, and to protect personnel.
In addition to reducing the TDS levels, all cooling water should be passed through a suitable deaerator and strainer installed before the water control valve. The deaerator will deoxygenate the water, thus reducing the potential of oxygen corrosion in the system.
The total installed length of a desuperheater station will vary with size and type, but it is typically about 7.5 m.
Most desuperheaters can be installed in any direction (the variable orifice type is a notable exception), but if installed vertically, the flow should be upwards.
The venturi type is best installed in a vertical pipe with the flow in the upward direction, as this aids mixing of the water and the steam. However, such installations are not usually possible due to the vertical space required.
Although it is possible to design desuperheater installations to operate with varying upstream pressures, it is much simpler if a constant supply pressure is maintained.
The amount of cooling water added is controlled by the temperature of the steam after the desuperheater. The higher the temperature, the more the control valve will open, and the greater the amount of water added. The target is to reduce the steam temperature to within a small margin of the design discharge temperature.
If the superheated steam supply pressure is increased, the saturation temperature will also increase.
However, the set value on the coolant controller will not change, and an excessive amount of water will be added, resulting in wet steam. Pressure sensors used in the control of the superheated steam pressure should ideally be located at the point of use, so that the pressure control valve can compensate for any line loss between the desuperheater and the point of use.
The minimum distance from the point of water injection to the temperature sensing point is critical:
The minimum installation distance will vary between different types of desuperheater and with different manufacturers. It is usually specified as a function of the temperature difference between
the required outlet temperature and either the inlet temperature or the coolant temperature.
Figure 15.4.2 shows a typical manufacturer’s sensor positioning chart.
Efficient drainage of the pipework following the desuperheater is essential. To ensure that water cannot accumulate at any point, the pipe should be arranged to fall approximately 20 mm per metre in the direction of flow, and should be provided with a separator station.
The steam trap used to drain the separator should be carefully selected to prevent air binding, and the discharge pipe from the steam trap should have ample capacity to deal with the drainage and it should be fixed as near to vertical as possible. In addition, there must be sufficient space in the drainage pipe for the water to flow down and air to pass up the pipe. The steam trap must also be able to withstand superheat conditions.
On critical applications, for example, prior to a turbine, a separator is even more important; the separator station will remove entrained water in the case of control failure, and prevent too much water being added to the steam.
To allow maintenance to be safely carried out, isolation valves are recommended upstream of:
Typically, these should be installed approximately, but no less than 10 pipe diameters from the item they are isolating.
A safety valve may be required to protect equipment downstream of the desuperheating station from overpressure, in the event of failure of the pressure control station.
It is necessary to ensure discharge pipework from the safety valve is led away to a safe area. This is of particular importance as high temperature superheated steam may be discharged.
Most equipment to be used on steam systems is designed with saturated steam in mind. It is therefore important that all equipment used in a desuperheater station will tolerate both the maximum temperature and pressure of the superheated steam.
Most equipment will have specified pressure and temperature limitations that are based on the nominal pressure (PN) rating of the material and the specific design of the device. By definition, the PN rating is the maximum pressure that a material can withstand at 120°C. For example, a PN16 rating means that the material will withstand a pressure of 16 bar g at 120°C. At higher temperatures, the maximum pressure will decrease, however, the exact relationship varies and depends on the material.
Figure 15.4.3 depicts typical pressure / temperature gradients for PN16, PN25 and PN40 rated products on a non-specific material. It is important to note that different materials will, by specification, produce variations in the temperature gradient.
In addition, components such as gaskets, fasteners and internal components may have a further limiting effect on the maximum temperature and pressure.
The selection and installation of the control devices to be used in a desuperheater station are an important consideration, as they can affect the overall turndown of the desuperheater. If the controls installed have a lower turndown ratio than the desuperheater itself, the turndown of the desuperheater station will be reduced (refer to Module 15.2).
Further information on basic control theory and practice can be found in Blocks 5 to 8 inclusive.
When selecting a suitable type of desuperheater for a particular application, the following factors need to be considered:
It is important to note here that, although ensuring that the device will have sufficient turndown for the flow likely to be encountered, it is important not to specify more turndown capability than is really needed. This predominantly affects cost, but it can also lead to poor system performance. Poor performance is often aggravated by the fact that most desuperheaters tend to perform better at the higher end of the specified flowrates and a system designer would tend to allow for increases in capacity due to expansion. As an extreme example, if the maximum flow specified was ten times the current requirement (in order to take into account future growth), the desuperheater will operate between 1 and 10% of its full flowrate instead of the 10% to 100% it is designed for.
Generally, where some degree of residual superheat can be tolerated, the desuperheated steam temperature should be as high above saturation as possible. This is beneficial for several reasons:
The method used to size a desuperheater will vary depending on the particular manufacturer and the type of desuperheater, and therefore it is outside the scope of this publication.
Desuperheaters are mainly applied in two areas: