Presentation on theme: "Steam and Water Accessories"— Presentation transcript:
1Steam and Water Accessories Chapter 4Steam and Water AccessoriesFeedwater Heaters • Feedwater Pumps • Surge Tank • Condensate Tank and Pump Unit • Main Feedwater Line • Steam Traps • Desuperheating and Pressure-Reducing Station
2The open feedwater heater, located above the feedwater pump on its suction side, supplies water to the pump at a slight pressure.The external connections consist of a float-controlled valve on the city water line, a condensate return line, an atmospheric vent line, and a low pressure steam line. Other connections include an outlet line to the feedwater pump, an overflow connection, and in some cases, an oil-separating device on the steam line. See Figure 4-1.
3Because of its vent condenser, the deaerating feedwater heater does a more effective job of removing oxygen and other noncondensable gases from the steam than an open feedwater heater.A deaerating feedwater heater is similar to an open feedwater heater. See Figure 4-2. The basic difference between the deaerating feedwater heater and the open feedwater heater is that the deaerating type has a vent condenser that separates oxygen and other gases from the steam before releasing the gases into the atmos- phere. The vent condenser is a shell-and-tube vessel located at the top of the heater.
4The horizontal deaerating feedwater heater is used when headroom is a problem. A deaerating feedwater heater is more efficient than an open feedwater heater. Deaerating feedwater heaters can also be horizontal in configuration. See Figure 4-3.
5The closed feedwater heater is located on the discharge side of the feedwater pump. Unlike the open feedwater heater, the closed feedwater heater does not mix steam and water.A closed feedwater heater is another type of feedwater heater found in some feedwater systems. See Figure A closed feedwater heater is basically a steel shell containing a large number of small tubes secured into two tube sheets. Closed feedwater heaters are designed for vertical or horizontal operation made in single or multiple passes. The manufacture and installation of a closed feedwater heater must allow for expansion and contraction of the shell and tubes.
6In an economizer, gases of combustion are used to raise boiler feedwater temperature. The gases of combustion are then released to the atmosphere.An economizer feedwater heater can be a unit type, which is outside the boiler casing, or an integral type, which is within the casing. These heaters have the gases of combustion passing around tubes through which the feedwater flows. See Figure 4-5. The increase in boiler efficiency can be substantial when using an economizer. Heat that would normally be lost is reclaimed directly from the gases of combustion.
7A two-stage condensing economizer recovers additional heat from the gases of combustion by preheating the incoming makeup water.Much of the heat can be recovered with a two-stage condensing economizer. See Figure 4-6. In the first stage, the two-stage economizer works like a standard economizer and recovers heat by preheating boiler feedwater with the heat from the gases of combustion. In the second stage, additional heat is recovered by preheating the incoming makeup water before it is fed to the deaerator. A boiler equipped with a two-stage condensing economizer and a high efficiency burner, and controlled by a fully integrated control system, can achieve over 90% fuel-to-steam efficiency and reduce greenhouse gas emissions by up to 10%.
8In a duplex-reciprocating feedwater pump, the steam piston must be 2 times to 2 1/2 times larger in area than the water piston.A reciprocating feedwater pump may be simplex (single cylinder) or duplex (double cylinder). The duplex-reciprocating feedwater pump is a simple type of steam-operated pump that runs at a slow speed. See Figure It is reliable, but does lose efficiency rather quickly when the steam valves or water valves are worn.
9In a centrifugal feedwater pump, the centrifugal force of the impeller produces pressure. The centrifugal feedwater pump is the most widely used feedwater pump today. In a centrifugal pump, the centrifugal force of a rotating element is converted into pressure. See Figure 4-8. Centrifugal pumps can be driven by an electric motor or by steam. These pumps have few moving parts.
10The two most common centrifugal feedwater pumps are the single-stage type and multiple-stage type. Many different types of centrifugal pumps are used on boilers. Generally, the two main types used are the single-stage type and the multiple-stage type. Single-stage centrifugal pumps are used for low to medium pressure steam boilers. Multiple-stage centrifugal pumps are designed for higher pressure steam boilers. See Figure 4-9.
11Turbine feedwater pumps are positive-displacement pumps and require an open discharge valve when starting.A turbine feedwater pump should not be confused with a centrifugal feedwater pump. See Figure Although they may look alike, the basic difference between the centrifugal feedwater pump and the turbine feedwater pump is in their design for discharging water.
12A surge tank provides the extra capacity required to handle changing loads and peak flows of condensate in larger steam boiler plants.In larger steam boiler plants, there are many sources from which condensate returns to the boiler room. In these plants, a surge tank is necessary to provide the extra capacity required to handle changing loads and peak flows of condensate. Normally, the surge tank is fitted with raw makeup water fittings and controls along with transfer pumps and controls. The surge tank is vented to the atmosphere and is equipped with a manhole for cleaning and repairs. See Figure 4-11.
13Condensate pumps are used to return all possible condensate from various parts of the plant to the open feedwater heater.In many steam boiler plants, condensate that is uncontaminated is pumped back to the open feedwater heater or a surge tank. The condensate is usually a considerable distance from the boiler room and cannot gravitate back. Condensate tanks and pumps are installed at the lowest points in each building where steam is being used for heating or process. See Figure 4-12.
14The main feedwater line utilizes check and stop valves to control water flow. The pipeline going from the discharge side of the feedwater pump to the steam boiler is the main feedwater line. This line must have a check valve and a stop valve located near the shell of the boiler. See Figure 4-13.
15When a feedwater pump feeds more than one boiler, each boiler should be isolated with a globe valve. The stop valve is required in case the check valve should fail and require repair or replacement. Without a stop valve, the boiler would have to be taken off-line for repair or replacement of the check valve. When two or more boilers are fed from a common source, a gate valve is installed on each branch line between the check valve and the main line. See Figure 4-14.
16The thermoexpansion feedwater regulator thermostat is located at the NOWL and is connected to the steam and water side of the boiler.The thermoexpansion feedwater regulator is connected to the boiler and the main feedwater line. See Figure The thermostat is located at the NOWL and is connected to the steam and water side of the boiler. If the water level drops, the steam space within the thermostat increases. This increases the temperature, causing the thermostat to expand.
17A two-element feedwater regulator has a diaphragm assembly on the top part of the regulator valve that senses the pressure differential from the superheater.For closer control of the water level in a boiler with a superheater, a thermoexpansion feedwater regulator can be equipped with a steam-flow sensing element. The element consists of a diaphragm assembly on the top part of the regulator valve that senses the pressure drop through an orifice device located in the steam line discharging from the superheater. This type of regulator is a two-element feedwater regulator. See Figure 4-16.
18A three-element feedwater regulator is used for closer control of water level in large watertube boilers with sudden fluctuations in steam load, and/or boilers equipped with economizers.A three-element feedwater regulator is used for closer control of water level in large watertube boilers with sudden fluctuations in the steam load, and in boilers equipped with economizers where there is a considerable drop in the feedwater pressure. A three-element feedwater regulator is more complicated than a one-element or two-element feedwater regulator. There are separate sensing elements for water level, steam flow, and feedwater flow. Information from the sensing elements is communicated using a transducer. A trans- ducer is a device that changes one type of signal to another type of signal. For example, the signal from the steam-flow sensing element is converted to a pneumatic signal. The pneumatic signal is then used to control overall boiler functions. See Figure 4-17.
19A thermohydraulic feedwater regulator operates when boiler water level increases or decreases. A thermohydraulic feedwater regulator consists of a regulating valve, bellows, generator, and stop valves. See Figure The control element is the generator, which is a tube within a tube. The generator is located at the NOWL and the inner tube is connected to the steam and water side of the boiler. Stop valves are used to isolate steam and water to the generator.
20Float feedwater regulators may be installed inside or outside the water column, depending on their design.A float feedwater regulator is used most often on small package boilers. This regulator consists of a float chamber, a float, and a mercury switch or a microswitch. See Figure The float chamber is connected to the steam and water side of the boiler. It is located at the NOWL. A blowdown line and valve remove any accumulation of sludge and sediment from the float chamber.
21Steam traps are located in the system wherever steam releases its heat and condenses. Steam strainers are installed before steam traps.Steam traps are located as necessary to achieve the most efficient removal of condensate. See Figure Several types of steam traps are available. These operate in different ways, yet they all perform the function of removing unwanted condensate and air from steam lines and heat exchangers. A steam trap must have a steam strainer installed on the inlet line. This will prevent scale or other solid particles from entering the trap.
22The thermostatic steam trap can be used with both high and low pressure steam. The thermostatic steam trap consists of a body, a valve, a valve seat, and a bellows-type element. See Figure When steam is in contact with the element, it causes the element to expand and the valve to close. If air or condensate surrounds the element, it contracts and the valve opens to allow the air or condensate to pass into the condensate return line.
23The float thermostatic trap removes air and other noncombustible gases as well as condensate. The float thermostatic steam trap has a body, a float that controls the valve, and a thermostatic element. See Figure When condensate enters the body of the trap, the float is raised and the valve opens to allow the condensate to flow into the condensate return line. If there is no condensate in the body, the valve will close.
24Inverted bucket steam traps are used when large quantities of condensate must be removed. The main parts of an inverted bucket steam trap are the body, the inverted bucket, the valve, and linkage that connects the valve and bucket. See Figure Steam and condensate enter the trap from the bottom of the inverted bucket. If only steam enters the trap, the bucket will become buoyant and the valve will close.
25An impulse steam trap valve opens when condensate or air entering the trap causes a pressure drop above the control disc.An impulse steam trap consists of a control chamber, a control disc, a valve, and a valve seat. The opening and closing of the valve depends on a pressure differential across the valve. When condensate or air enters the trap, a pressure drop occurs above the control disc, lifting the valve off the valve seat to open the valve. Air and condensate enter the return line until steam enters the trap, increasing the pressure above the control disc to close the valve. See Figure 4-24.
26In a thermodynamic steam trap, pressure of the air or condensate enters the inlet port under the center of the disc and lifts the disc from its seat to discharge condensate.A thermodynamic steam trap consists of a body, a control chamber, a cap, and a movable disc. Pressure of the air or condensate enters in the inlet port under the center of the disc and lifts the disc from its seat. The discharge of condensate continues until the flashing condensate approaches steam temperature. The high velocity steam under the disc recompresses, building up pressure in the control chamber above the disc. This pressure forces the disc down to the seat, ensuring a tight closure without steam loss. Steam pressure in the control chamber acting on the total disc area from above holds the disc closed against the inlet pressure from the smaller inlet seat below. As condensate collects in the trap, it reduces heat transfer to the control chamber. The pressure in the control chamber decreases as the steam in the control chamber condenses. The disc is then lifted by the inlet pressure and the condensate is discharged to repeat the cycle. See Figure 4-25.
27In a variable orifice (labyrinth) steam trap, the steam and condensate enter the body and pass through a series of compartments that reduce the pressure.The variable orifice (labyrinth) steam trap consists of a body, a series of compartments, a sight glass, and an adjustable valve. The steam and condensate enter the body and pass through a series of compartments that reduce the pressure. In the first compartment, the condensate level can be seen with the sight glass. The manual orifice is adjusted to maintain a constant condensate level in the sight glass. See Figure Some variable orifice steam traps automatically open and close because they have a wax that expands when exposed to steam and contracts when exposed to condensate.
28Drum desuperheaters are used primarily on marine boilers. To desuperheat superheated steam, enough heat must be removed to bring the steam back to the saturation point. One method of desuperheating steam, which is used in marine boilers, is to route steam back through coils of submerged piping within the steam and water drum. See Figure 4-27.
29The quantity of steam increases with the use of a line desuperheater. Another type of desuperheater is the line (spray) type. It is used in conjunction with a pressure-reducing station. See Figure This desuperheater has a chamber in the steam line that is fitted with one or more nozzles. The nozzles deliver a fine spray of feedwater into the superheated steam, thus reducing the temperature. The quantity of steam increases when the feedwater changes state after absorbing the superheat. A thermostat is placed in the line on the saturated steam side of the desuperheater. This controls the feedwater supply valve to the nozzles. If the temperature of the steam rises, the feedwater supply valve to the nozzles will begin to open. If the temperature of the steam falls, the feedwater valve will start to close.
30Plants often require steam at different pressures Plants often require steam at different pressures. Steam at the required pressure is supplied by using pressure-reducing stationsPlants often require steam at different steam pressures for different purposes. For example, high pressure steam may be required for process and low pressure steam for heating, cooking, and laundry operations. A pressure-reducing station reduces steam pressure as the steam passes from the high side to the low side. See Figure The low pressure steam cannot dissipate all of its heat immediately to reach the temperature corresponding with its pressure and becomes superheated steam. The steam eventually reaches its corresponding temperature further downstream. This increases efficiency when directing steam to remote loads as the steam will not condense as readily as saturated steam. It also allows for the piping and equipment on the low pressure side to be designed much lighter, saving installation costs.