Steam Boiler: Pump

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Control Valve Characteristics

Control valve is a type of final control element that is most commonly used for process control systems. Control valve serves to regulate flow of fluid so it can be determined in accordance with controller's requirement. Valve has important role in steam boiler.

Most of control valve characteristics that are used in industrial process are as follow:

Linear Characteristic

The control valve will give increment value of flow rate close to each increment travel (proportional).

Equal percentage characteristic

The percentage of valve opening (valve travel) will give same percentage of amount of flow rate.

Quick opening characteristic

Control valve provide maximum change in flow with small valve opening and maintain linear relationship at all positions of fluid movement. Each additional valve opening provides sharp change in flow rate and when the valve approaching full opening position, flow changes near zero.

A control valve consists of two parts, actuator and valve as shown in Figure 2.5 below:

Figure 1: Control Valve

Actuator part is the part that moves to perform on open / close valve. This part is widely used is pneumatic operated (diaphragm), electric actuators, hydraulic actuators, and manual / hand operated actuator. Spring and pneumatic diaphragm actuator are widely used because of the ability and its structure is simple.

Valve Parts are mechanical components that determine amount of flow that goes into process. In its unity as the unit control valve, actuator and valve must perform correction based on manipulated variable signal which coming out of controller.

Centrifugal Pumps

Definition of pump is machine that is widely used to flow incompressible fluid from low place to higher place or low pressure to higher pressure. When viewed from pressure that generates fluid energy, pump can be classified into two types:

Static Pressure Pump
Dynamic Pressure Pump

Centrifugal pumps are included into the type of dynamic pressure pump. Centrifugal pumps have impeller that serves to lift fluid from low place to higher place or flow fluid from low pressure to higher pressure.

Power from outside is supplied into the shaft to rotate impeller in pump house, so fluid around impeller will also be spun as a result of encouragement impeller blades. Because of centrifugal force occurs, the fluid flows from center of impeller exit through channels between impeller blades. Fluid head will be greater because the fluid is accelerated. Fluid coming out of impeller will be accommodated by channel-shaped volute around impeller and fluid is channeled out by pump through nozzle, in the nozzle fluid flow velocity is converted into pressure head.

Characteristics of centrifugal pumps are determined by quantities such as capacity, high-pressure fluid, properties or state of suction side, power which is needed to rotate pump, rotary speed, and efficiency. The amount of power which is needed by centrifugal pumps is function of coupling that is given by motor that can be measured by several methods, such as by using brake dynamometer, weirmeter and Prony brake method.

In the process for setting the capacity of a pump installation or change the characteristics (setting by opening the valve) can be done, but this applies if the deviation required capacity of the nominal capacity is only valid within a short time, and when the engine power needed to pump drive capacity smaller become increasingly down (reduced and not increased up).

Setting capacity of pump installation or change characteristics of pump by setting opening valve can be done. However this can be performed when deviation of capacity which is required form nominal capacity is within short time and when the engine of power which is needed to pump is smaller and only need small capacity.

Net Positive Suction Head Required (NPSHR) of Pump

Pump which is used for power plant can be called as Boiler Feed Pump. Boiler feed pump serves to supply feedwater into steam boiler. Pump science is the interesting knowledge to be learned. One of them is about Net Positive Suction Head (NPSH) of pump.

The lowest pressure within the pump can usually be found at a point near the side entrance impeller blades in that place, the pressure is lower than the pressure at the suction pump hole. This is due to head losses in the suction nozzle, the flow velocity increases due to cross-sectional area narrowing, and increased flow velocity due to thickness of local blade.

To avoid evaporation of liquid, the pressure on pump inlet is reduced with a decrease in pressure within pump should be higher than steam pressure of liquids. A large pressure head equal to pressure drop is called as NPSHR / Net Positive Suction Head Required. The amount of Net Positive Suction Head Required is different for each pump. For a particular pump, NPSH required changes according to capacity and rotation.

In order for the pump can work without experiencing cavitation, it must meet NPSH available is greater than the NPSH required. NPSH required value must be obtained from the pump manufacturer. But for a rough assessment, the NPSH required can be calculated by the following equation:

σ = H_svN / H_N

Where:

σ = coefficient of Thoma cavitation

H_svN = NPSH required (m)

H_N = total head pump at the point of maximum efficiency (m).

The suction specific speed (S) can also be used as a replacement for coefficient of Thoma cavitation in calculating NPSH required. The correlation can be seen in the following equation:

H_svN = (n/S)^4/3 . Q_N^2/3

Where:

H_svN = NPSH required (m)

n = pump rotation (rpm)

Q_N = pump capacity (m³/min)

S = the suction specific speed (m / min).

Net Positive Suction Head Available (NPSHA) of Pump

Net Positive Suction Head Available (NPSHA) is the head which is owned by the liquid at the pump suction side which is reduced with the saturated steam pressure of liquid in that place. In the case of pump that sucks liquid from the open place, then the amount of available Net Positive Suction Head Available (NPSHA) can be written as follows:

h_sv = (Pa/γ) - (Pv/γ) – h_s – h_ls

Where:

h_sv = NPSH available (m)

Pa = atmospheric pressure (kg/m2)

Pv = saturated steam pressure (kg/m2)

γ = weight of liquid per unit volume (kg/m3)

h_s = static suction head (m), hs is positive (marked +) if the pump is located above the liquid surface, and negative (marked -) if below the liquid surface.

h_ls = head loss in suction pipe (m).

If the liquid is sucked from closed tank, then the Pa value states absolute pressure which acting on the surface of the liquid inside closed tank. Especially if the pressure above the liquid surface is equal to saturated steam pressure, then Pa = Pv. In the case of a pump that sucks the liquid from the open, so the amount of Net Positive Suction Head Available (NPSHA) can be written as follows:

h_sv = – h_s - h_ls

The value of hs is negative (-) because the surface of the liquid inside the tank is higher than the suction side of pump. Installation of this kind pump is required to obtain a positive NPSHA value.

Figure 1: Position of pump is located above the surface of sucked fluid

Figure 2: Position of pump is located under the surface of sucked fluid

Net Positive Suction Head (NPSH) of Pump

Pump cavitation will occur when the static pressure of a flow down to below the saturated steam pressure. To avoid pump cavitation endeavored not to have any part of the flow inside the pump which has a lower static pressure than the saturated steam pressure at a temperature of the liquid.

In this case need to be considered that there are two kinds of pressure which plays important role. First, the pressure is determined by environmental conditions in which the pump is installed, and the second, pressure is determined by the state of flow within the pump.

Related to two things above, can be defined a Net Positive Suction Head (NPSH) which is used as a standard to measure the safety of pump from cavitation. There are two kinds of Net Positive Suction Head (NPSH) as the following below:

1. Net Positive Suction Head Available (NPSHA) of pump

2. Net Positive Suction Head Required (NPSHR) of pump

The way to avoid pump cavitation is the NPSHA (Net Positive Suction Head Available) is greater than the NPSHR (Net Positive Suction Head Required).

Pump Cavitation

Feedwater is supplied into steam boiler by boiler feed pump. In the operation of boiler feed pump, there is possibility some event may occur which cause damage pump parts. One of these cases is pump cavitation.

Pump cavitation is the formation events of steam or vapor bubbles within the liquid caused by the drop in fluid pressure to below the saturated vapor pressure of liquid at the operating temperature of the pump.

Vapor bubbles are formed in this process has a very short cycle. Knapp (Karassik et al, 1976) found that the starting formation of the bubble until the bubble burst only takes about 0.003 seconds. These bubbles will be carried by the flow of fluid until and finally these bubbles are in the region that has a greater pressure than the saturated vapor pressure of liquids.

In that area, the bubble will burst and will cause the shock to wall nearby of pump. The liquid will enter suddenly into the room which are formed due to the rupture of vapor bubbles was causing the collision. This event is called as pump cavitation and will cause mechanical damage to the pump.

Figure 1: Pump Cavitation

Specific Speed Pump

Boiler feed pump has important role to supply feedwater into steam boiler. Many parameters should be considered such as centrifugal pump type, performance pump, capacity pump, specific speed pump, head pump, rotation pump, and so on. Capacity of pump must be calculated based on requirement of boiler water which is needed to supply steam into turbine generator.

Performance of centrifugal pumps (except regenerative turbine generator) is connected to a parameter which is called the specific speed pump. As defined by the Hydraulic Institute, specific speed pump is relationship between capacity, high-press, and the speed at optimum efficiency which classifies the pump impeller with respect to the geometric equation. Specific speed pump is an algebraic number is expressed as:

Ns = N . (Q^0.25 / H^0.75)

Where:

Ns = specific speed pump (m/min)

N = rotation pump (rpm)

Q = pump capacity (m³/min)

H = total head pump (m)

Head Pump

Head Pump is the energy per unit weight that should be provided to flow liquid which is planned in accordance with the conditions of the pump installation, or the pressure to flow some amount of fluid, which is usually expressed in length units. According to Bernoulli equation, which states that “when incompressible fluid flowing along the pipe and its cross section has a different height, the pressure difference depends not only on the difference in height but also on the difference between velocity at each point ".

In the Bernoulli equation, there are three kinds of head pump (energy) flow of fluid from the installation system, namely, pressure energy, energy potensial and kinetic. This condition can be expressed as the following formula: (Bruce Munson, 2006)

H = (P/γ) + Z + (V²/2g)

Where:

H = Total head pump (m)

P/γ = pressure head pump (m)

Z = Total static head pump (m)

V²/2g = Velocity head pump (m)

Because of energy is conserved, so the head (high press) may vary in different sections. But in fact there are always losses of energy.

Figure 1: Scheme of Head Pump

Caption:

1 = suction reservoir

2 = suction pipe

3 = pump

4 = press pipe

5 = press reservoir

In different conditions as in the figure above, the Bernoulli equation is as follow:

(P_A/γ_A) + (V_A²/2g) + Z_A + H = (P_B/γ_B) + (V_B²/2g) + Z_B + H_L (Loss A to B)

H = ((P_B/γ_B) - (P_A/γ_A)) + ((V_B²/2g) - (V_A²/2g)) + (Z_B - Z_A) + H_L

Because of γ_A = γ_B so:

H = ((P_B - P_A)//γ) + ((V_B² - V_A²)/2g) + (Z_B - Z_A) + H_L

H = (ΔP//γ) + (ΔV²/2g) + H_ST + H_L

Where:

H = total head pump (m)

ΔP/γ = head pump because of different pressure on suction side with press side (m)

ΔV²/2g = head pump which is caused by different velocity (m)

H_ST = static head (m)

H_L = heat loss from A to B (m)

Centrifugal Pump Curve

Centrifugal pump is one of pump type. it has some classifications based on certain parameter. It is important to know centrifugal pump parts and its function. Furthermore characteristic and working principle of centrifugal pump can be well understood.

Characteristics of centrifugal pumps are a way in which the high tap differential pressure varies with the output at a constant speed. Characteristics of centrifugal pump may also include the efficiency curve and the amount of its brake horse power.

High-capacity curve pressure as shown in Figure 1 below is described as an increase in total high-capacity pressure, where the high pressure pump is able to be raised or decreased. Generally centrifugal pump curve will raise amount of pressure at greatest point where no flow is often regarded as a shut off head.

If a shut off head is less than the maximum amount of the high pressure, the pump becomes unstable and under some conditions can increase the power and speed fluctuations that cause large mechanical vibrations in piping systems.

Figure 1: Centrifugal Pump Curve
(Source: Biro efisiensi energi, 2004)

Centrifugal Pump Parts

Pump in the power plant is utilized to flow feedwater into steam boiler. The name of this pump is Boiler Feed Pump (BFP). Type pump which is often used is centrifugal pump. Centrifugal pump has some parts. The following below are the parts of centrifugal pump:

Figure 1: Centrifugal Pump Parts

Stuffing Box

Stuffing Box serves to prevent leakage in the area where the shaft penetrates the pump casing.

Packing

Packing is used to prevent and reduce the leakage of fluid from the pump casing through the shaft. Packing is usually made of asbestos or Teflon.

Shaft

Shaft serves to continue the torque from the drive during operation and the seat of the impeller and other rotating parts.

Shaft sleeve

Shaft sleeve serves to protect the shaft from erosion, corrosion and wear of the stuffing box. In multi-stage pump can be as leakage joint, internal bearings and inter stage or distance sleever.

Vane

Vane is blades of the impeller as a place of passage of fluid in the impeller.

Casing

Casing is the outermost part of pump that serves as a protective element that rotates, the seat of diffusor (guide vane), inlet and outlet nozzle as well as a place to give flow direction from the impeller and convert energy of fluid velocity into dynamic energy (single stage).

The Eye of Impeller

The eye of impeller is the entrance side of suction direction of impeller.

Impeller

Impeller serves to convert mechanical energy of the pump into velocity energy speed of fluid which is pumped continuously, so that the liquid on the suction side can continually fill the empty caused by the displacement of fluid that entered previously.

Wearing Ring

Wearing the ring serves to minimize leakage of fluid passing through the front of impeller and the back of impeller, by minimizing the gap between the impeller casing.

Bearing

Bearing serves to withstand the load shaft so it can rotate continuously, either in the form of radial loads and axial loads. Bearing also allows the shaft to rotate smoothly and remain in place, so that frictional losses become smaller.

Centrifugal Pump Classification

Centrifugal pump can be classified based on some parameters. The following below are the parameters of centrifugal pump classification:

Based on discharge pressure:

Low pressure pump, the operating pressure less than 5 kg/cm²

Medium pressure pump, the operating pressure approximately 50-50 kg/cm²

High pressure pump, the operating pressure higher than 50 kg/cm²

Based on capacity:

Low capacity pump, the capacity less than 20 m³/h

Medium capacity pump, the capacity pump approximately 20-60 m³/h

High capacity pump, the capacity higher than 60 m³/h

Based on direction of exit flow of impeller:

Radial flow pump

Axial flow pump

Mixed flow pump

Based on number of suction

Single suction pump

Double suction pump

Based on shaft position:

Vertical shaft pump

Horizontal shaft pump

Based on number or composition of impeller and level:

Single stage pump, consists of one impeller and one casing.

Multiple stage pump,

Multiple impeller pump, consists of several of impellers which is arranged parallel in a single casing.

Multiple impeller – multiple stage pump, the combination of multiple impeller pump and multiple stage pump.

Pump Definition

Pump is the equipment to move fluid (liquid) from one point to other point by increasing pressure of that liquid. Increasing the pressure of liquid is used to overcome flow resistance. The flow resistances can be caused by difference pressure, different height or friction resistance. Pump can be generally classified as two types, namely: positive displacement pump and non positive displacement pump. One of non displacement pump type is centrifugal pump which has working principal to convert kinetic energy of liquid to potential energy (dynamic) through an impeller which rotates in the casing.

In the steam boiler, type pump which is mostly used is centrifugal pump type because its simplicity and low cost. Steam boiler need feedwater pump to supply water into steam drum. Centrifugal force is a force which occur because a particle move through circle way. The advantage of centrifugal pump than type positive displacement pump is the impeller can run continuously so the flow will be soft and do not produce bubble. Other advantages of centrifugal pump are can operate in high speed rotation, can be coupled electric motor or small scale steam turbine so only need small and light room, and low cost for installation and maintenance.