Working principle and application of temperature and pressure reducing device

I. Working Principle of Temperature and Pressure Reduction Device

When the supply of high-temperature and high-pressure superheated steam transported to the steam point, it must first enter the decompression and temperature reduction device, the superheated steam pressure and temperature down to close to the required saturation (generally close to the saturation temperature of 3-5 ℃), and then sent to the folding into the use of heat transfer equipment.

     There are two most basic forms of desuperheaters.

1. non-contact

 The medium cooling the steam is not in direct contact with the superheated steam being cooled. Cooler liquids, gases and vapors can be used as the cooling medium. The surrounding air can also be used as a cooling medium. This type of desuperheater is like a shell-and-tube heat exchanger. The superheated steam enters one side of the heat exchanger and the cooling medium enters the other side of the heat exchanger. When the temperature of the superheated steam is controlled, it is possible to regulate: the flow rate of the inlet superheated steam, or the flow rate of the cooling medium.

 2. Direct contact

 The medium used to cool the steam (usually water) is mixed directly with the superheated steam, as shown in the following diagram of the Venturi and direct injection type temperature and pressure reduction system.

        The superheated steam is first depressurized and enters the pressure reducer. Cooling water is mixed directly with the superheated steam, absorbing heat from the superheated steam and evaporating it into steam. In turn, the superheated steam is cooled. A certain amount of cooling water is added through an atomizing and mixing device inside the desuperheater. The amount of cooling water added is controlled by measuring the steam temperature downstream of the desuperheater. So dry steam can be produced. This prevents damage and erosion of the underlying piping and equipment.

      All direct contact desuperheaters must break the incoming water into small droplets to increase the surface area/volume ratio of the water. The greater the surface area/volume ratio of the water, the faster the droplets evaporate and the faster the steam cools down. The process of creating small droplets is often referred to as "atomization". Sprayed into the desuperheating water atomization quality is good or bad, will directly affect the control performance of the desuperheating system, different types of desuperheaters using different desuperheating water atomization methods.

       It is worth drawing attention to the materialization of water droplets mixed with steam, water droplet evaporation (while the steam cooling) is a process that takes time, will not be completed instantly. As a result, most of the desuperheating process does not occur inside the desuperheater, but in the piping downstream of the desuperheater outlet. Therefore, the design of the piping downstream of the installation is also critical to a good smell up.

       It is easy to understand why cooling water droplets and superheated steam need a period of time to mix well. If the mixing is poor, the water can not effectively absorb heat from the superheated steam, the evaporation process of the water droplets will not be complete, resulting in water droplets overflowing downstream of the desuperheater, and the temperature at the outlet of the desuperheater can not be controlled. Therefore, the water droplets should remain suspended in the downstream piping for as long as possible. To ensure this, the downstream piping should be maintained at a relatively high flow rate to maintain sufficient turbulence in the downstream piping. This velocity should be higher than the steam flow rate of a typical steam distribution system. This is why desuperheaters and corresponding piping are usually (not always) smaller than steam distribution system piping.

The usual choices of water sources for cooling are: boiler make-up water, demineralized water, deionized water, and condensate.

Municipal tap water or process water may also be used, depending on the hardness of the feed water. Scale may accumulate inside the desuperheater cooling water nozzles and on the inside surfaces of the piping downstream of the desuperheater.

Typically, the higher the cooling water temperature, the better, due to the fact that hot water droplets absorb less heat than cold water droplets to reach evaporation temperature and therefore evaporate faster, resulting in more efficient desuperheating. The use of hot water also reduces the amount of water falling onto the inside walls of the pipes. Feed pipes should therefore be insulated. Pressure drop is required through the water control valve. We said earlier that the water should be as hot as possible, but flash evaporation through the control valve should be avoided.

In order to inject cooling water, the pressure at the desuperheater nozzle must be equal to or greater than the pressure of the operating steam in the piping. Different types of desuperheaters have different pressure requirements but the usual minimum pressure values are as follows:

Jet type desuperheater Steam pressure +0.5bar

Venturi Type Desuperheater Steam Pressure +0.1bar

Steam atomizing desuperheaters Same as steam pressure

For jet and venturi type desuperheaters, the maximum pressure is required at the maximum water flow rate. It should be noted that the water flow rate is proportional to the square root of the pressure difference between cooling water and steam. Thus if the water flow rate is increased by a factor of 4, the differential pressure has to be increased by 42 = 16.

If a stand alone or booster pump is used, a return system is required to ensure that water is always passed through the pump.

Calculating Cooling Water Consumption

Sufficient water must be added to cool the steam to the desired temperature; if there is not enough water the steam will not be cooled sufficiently, and if there is too much water, wet saturated steam may be produced. This can lead to erosion of downstream piping and equipment.  The following enthalpy balance equation can be used to easily and quickly calculate the amount of cooling water required:

 
 Ms x(hi—hd)=Mw x(hd-hw)
 
 Mw=[Ms X(hi-hd)]/(hd-hw)
 

where

 Mw=amount of coolant kg/h

    Ms=amount of superheated steam kg/h

 hi=enthalpy of superheated steam kJ/kg

  hd=Enthalpy of steam in reduced temperature state kJ/kg

 hw=enthalpy of coolant kJ/kg

Structure and type of desuperheater

The desuperheater should compensate for changes in environmental conditions, steam temperature or flow rate. Its selection depends on the following factors:

u Operating pressure

u Operating temperature

u Steam flow rate

u Superheat before and after desuperheating

u Required regulation ratio

u Water pressure available (a booster pump may be required if sufficient pressure is not available)

u Required control accuracy of the final temperature

There are 3 different types of desuperheaters, each designed to meet the specific process needs based on the process data on site:

(1) Jet Type Desuperheater, (2) Venturi Type Desuperheater, and (3) Steam Atomization Type Desuperheater.

 The structure and characteristics of the different types of desuperheaters are described below.

1、Venturi type desuperheater

Venturi type desuperheaters utilize throttling to create high velocity zones and turbulence to help achieve complete contact between steam and cooling water for maximum desuperheating. The desuperheating process is accomplished through three stages:

The first stage of desuperheating occurs within the internal diffuser. A portion of the steam is accelerated within the internal nozzle and atomizes the incoming water.  The second stage of desuperheating is the mixing of the saturated mist from the internal diffuser with the remaining steam in the main diffuser. The main diffuser itself generates high velocity by restricting the flow of the steam, thus creating strong turbulence in the area and completing the second stage of desuperheating. The third and final stage of desuperheating occurs in the piping downstream of the outlet of the desuperheater. During this stage the remaining water droplets suspended in the steam evaporate. The final required temperature is reached at a point downstream of the desuperheater.

This device minimizes the possibility of contact between the cooling water and the inner wall of the piping. Minimal pipe erosion and maximum desuperheating effect. The Venturi desuperheater has the greatest high-speed mixing effect. The steam flow regulation ratio varies according to the actual working conditions, and is generally 4:1 for horizontally mounted desuperheaters, and can reach more than 5:1 for vertically mounted desuperheaters. when used in conjunction with well-designed depressurization stations, steam flow regulation ratios can be improved to more than 5:1.

If the steam flow regulation ratio is beyond the capability of a single desuperheater, two desuperheaters can be used in parallel and automatically switched in response to changes in steam flow.

Features of Venturi Type Desuperheater

Adjustment ratios generally meet most plant applications, and for most applications, pressures are relatively low.

Simple operation. No moving parts.

Precise control, usually up to saturation temperature TsAT+3°C.

Suitable for applications with stable or variable steam conditions.

2、Complete Jet Type Desuperheater

A complete jet-type desuperheater is easy to install. It includes a jet nozzle assembly, heat sleeve and flange connection shell sleeve.

This is the simplest type of desuperheater, the coolant is sprayed into the steam stream through one or more atomizing nozzles. The atomizing nozzles are located on the central axis of the desuperheater and the coolant is sprayed into the steam in the same direction as the steam.

The desuperheater includes a heat sleeve. The superheated steam can pass through an annular surface between the heat sleeve and the inner diameter of the tube casing. The heat sleeve provides a hot surface that allows the injection fluid to evaporate quickly while protecting the shell of the desuperheater from erosion. The work of the heat sleeve allows the desuperheater to ensure efficient operation of the system when the system is under low load and the atomization of the nozzles is not at its most efficient.

Features of spray-type desuperheater

Simple operation. No moving parts.

Low cost.

Zero vapor pressure drop.

Low flow regulation ratio capability

 Low ability to approach saturated steam temperature (TsAT+5°C is generally preferred).

Tend to cause erosion of the inner wall of the steam piping. The use of internal thermal casing can overcome the problem． It also helps the evaporation of moisture.

Applications: more stable steam load, more stable steam temperature, more stable cooling water temperature

3, steam atomization type desuperheater

This type of desuperheater using auxiliary high-pressure steam in the desuperheater diffuser to enter the cooling water for atomization. Auxiliary steam pressure is at least 1.5 times the pressure of the desuperheater inlet steam pressure (gauge pressure). The minimum pressure requirement of 3 barg. In general, the atomized steam flow rate of the main steam flow rate of 2% to 5%. Desuperheating process is completed in two stages.

The first process is completed in the diffuser, cooling water is atomized by high-speed atomized steam. In the second stage of desuperheating, the saturated mist from the diffuser is mixed with the steam in the main pipeline. The evaporation process takes place in the outlet pipe of the desuperheater. The moisture remaining in the outlet pipe is suspended in the steam and gradually evaporates so that the final required temperature is reached at a point downstream of the desuperheater.

Features of Steam Atomization Type Desuperheater

The coolant is introduced from the center of the main steam flow.

The coolant is sprayed eight steam in the direction of the steam flow.

Good regulation ratio. Adjustment ratios of up to 50:1 are possible for steam flow, but the most efficient operation and control is achieved with a ratio of about 20:1. The same data applies for the coolant.

The structure is very compact and has the shortest length of all the desuperheaters.

Negligible pressure drop

Suitable for applications where the steam flow rate is highly variable and a high regulation ratio is required.

Installation Orientation of Desuperheater and Control Components

The desuperheater can be installed horizontally or vertically. When installed vertically, steam must flow upwards. Vertical installation requires the steam to flow upward. Spirotech strongly opposes vertical installation where the steam flows downward. For horizontal installation of the desuperheater. The ideal installation direction is with the cooling water connection downwards (the same applies to the atomized steam connection for steam atomized desuperheaters). Satisfactory operation can also be achieved in other directions, but drainage is less effective.

For vertical installation. We recommend that the cooling water piping (and, if required, the atomizing steam piping) should be connected from below to the corresponding connections of the desuperheater. This arrangement ensures optimum drainage after shutdown.

Distance between desuperheater and pressurization valve

In pressure- and temperature-reducing applications, the desuperheater should be located at least 5 times the pipe passage or 1.5 m downstream of the pressure-reducing valve (distance A in the system diagram).

The pressure sensor should be located at least 1.5 meters downstream of the desuperheater outlet flange. Ideally, however, the pressure sensor should be located at the point of steam use. This allows the pressure control valve to compensate for any piping pressure loss between the desuperheater and the point of use.

The distance between the water spray point and the temperature sensor is critical. If the sensor is too close to the spray point, the water will not evaporate sufficiently and the temperature sensor will give a false reading. The location of the temperature sensor depends on many factors. The most important of these is the value of the residual superheat.

It is important to maintain a constant supply vapor pressure.

The steam temperature after the desuperheater controls the amount of water added. The higher the temperature, the larger the control valve opens and the more water is added. Normally the goal of desuperheating is to reduce the temperature of the superheated steam to a very small range above the saturation temperature of the steam. However. If the supply steam pressure rises, the corresponding saturation temperature also rises. The corresponding saturation temperature also rises. If the controller's setpoint does not change, the control system will add additional water to achieve the setpoint, which will result in very wet steam, which can cause many problems.

In some applications where steam must not carry water, it is recommended that a vapor separator be installed downstream of the desuperheater. This protects downstream piping and equipment from moisture during control system failures or abnormal operating conditions such as start-up. A separator is also recommended downstream of the desuperheater for applications where the temperature of the steam after desuperheating is close to saturation or for applications with large turndown ratios (e.g., turndown ratios >2:1 for water-injected desuperheaters, >3:1 for Venturi desuperheaters, >5:1 for steam-atomized desuperheaters).  The separator must be installed downstream of the temperature sensor so that the water has sufficient time to evaporate. The trap used in conjunction with the separator should prevent air clogging, and the discharge pipe from the trap should have sufficient capacity to discharge the cooling water and be mounted vertically as close as possible.

 It is recommended that a strainer be installed on the cooling water supply line to protect the control valve and desuperheater from clogging. It is also recommended that a strainer be installed upstream of the superheated steam pressure control valve.

 In applications involving pressure reduction control, a safety valve should be installed downstream of the pressure reducing valve to protect the desuperheater and downstream equipment in the event of the following conditions

 Overpressure in the event of a pressure control system failure

 over-temperature in the event of a pressure control system failure.

The desuperheater and downstream equipment must be capable of meeting the maximum temperature limits of the superheated steam. To ensure system safety in the event of pressure and temperature control system failure.

When considering the entire system, it should be kept in mind that the desuperheater is only one component of the temperature reduction system. Obviously, if the installed control system has a lower regulation ratio than the desuperheater, the regulation ratio of the entire desuperheating system is also reduced. For example, in a particular desuperheating and depressurization system, if the cooling water control valve has a lower ratio than the desuperheater, the cooling water control valve ratio will limit the ratio of the entire desuperheating and depressurization system.