Drum Lvl Control by Pkgangwar

Instrumentation Limited, Kota( A Government of India Enterprise )

FEED WATER FLOW CONTROL

1. MEASUREMENT OF DENSITY CORRECTED DRUM LEVEL Page 1-4

2. FEED WATER FLOW CONTROL Page 5-8

a) BY FEED WATER FLOW CONTROL VALVE.i. 1-ELEMENT CONTROLii. 3-ELEMENT CONTROL

b) BY BFP SCOOPi. DP CONTROL ACROSS F W C Vii. 3-ELEMENT CONTROL

3. BFP PROTECTION CONTROL Page 9-10

4. BFP RECIRCULATION CONTROL Page 11

PROJECT :

NTPC / NCPP, DADRI (4 x 210 MW)

BY : P K GANGWARSr. Manager (Power Project Group)

Instrumentation Limited, Kota

MEASUREMENT OF DENSITY CORRECTED DRUM LEVEL

In order to have balance between steam flow and feed water flow , accurate measurement of Drum Level and it’s control at fixed set point is necessary. Boiler Drum in Power Plant is a high pressure and high temp. vessel . Typically in a 210 MW Power Plant, Drum Pressure and Temp. are of the order of 160 Kg/cm2 and 345 deg. C. respectively. The water and steam in the drum are in saturated condition. At such a high pressure and temp. , there is significant change in density of water and steam . Typically, at above pressure and temp., density of water is approx. 0.59 and that of steam is approx. 0.106.

The measurement of drum level is done indirectly by measuring Differential Pressure generated between a constant reference head and a variable head dependent upon the actual level in the drum. As the Diff. Pressure is affected by the changing density of water and steam , this become necessary to compensate the measured Diff. Pressure for density changes in drum water and steam .

Density of water and steam in the Boiler Drum , depend upon the pressure and temp. of the medium inside the drum. Where as the density increases with increase in pressure , it is inversely proportional to temp. Depending upon , how the effect of temp. is compensated, there are two methods of measurement as described below :

NTPC Dadri - 1 -


NTPC Dadri - 2 -

Saturated Water & Steam Properties

0.000000

0.200000

0.400000

0.600000

0.800000

1.000000

1.200000

0.000000 50.000000 100.000000 150.000000 200.000000 250.000000Pressure Abs Kg/cm2

Sp

ec

ific

De

ns

ity

gm

/cc

Water

Steam

dw-ds


MECHANICAL TEMP. COMPENSATION

A temperature compensated dual column Constant Head Unit (CHU) is used, where continuous flow of water, condensed from steam, is maintained into the drum in order to keep the CHU temp. same as that of drum. The faster is the condensation, the temp. of CHU is more nearer to that of drum. This ensures that the density of water & steam , and thereby level in the variable column is almost the same as that water & steam inside the drum . Pressure compensation is applied either mechanically by using a cam or electronically through a multiplier or divider circuit . This type of density compensation was used in RUSSIAN & G.K. system of as illustrated below :

a) RUSSIAN SYSTEM

STEAM ds

NWL H

WATER dw h

DP = H dw – ( h dw + ( H-h) ds) = H dw- h dw – (H-h) ds = (H-h) dw – (H-h) ds = (H-h) (dw-ds)

This results into the following equation :

NTPC Dadri - 3 -

(H-h) = DP/(dw-ds)


b) GEORGE KENT SYSTEM

k

STEAM ds

NWL H

WATER dw h

DP = (H+k) dw – [ (H+k-h) ds + h dw)] = H (dw – ds) – h (dw-ds) + k (dw – ds) = (dw – ds) [(H-h) + k]

This results into the following equation :

The purpose of excess length of CHU is to ensure faster and greater condensation of saturated steam into water and thus ensuring that the temp. of water inside the CHU is more nearer to that of drum water . The excess +ve head of `k’ is nullified in the electronics .

NTPC Dadri - 4 -

(H-h) = DP/(dw-ds) - k


ELECTRONIC TEMP. COMPENSATION

Temp. and Pr. compensation is done in the electronic circuits using multipliers, dividers & function generators, as has been done in CONTRONIC system of M/s. H & B Germany :

CONTRONIC SYSTEM

Dw’ dw

STEAM ds

NWL H

WATER dw h

DP = H dw’ – [h dw + (H – h) ds] = H (dw’ – ds) – h (dw - ds) This results into the following equation :

Here, dw’ is the average density of water in the Reference Column , which differ from place to place depending upon the climatic conditions . For ease of calculation , four Reference Temperatures have been considered , viz, 25, 50, 75 & 100 deg. C.

NTPC Dadri - 5 -

h = [H (dw’- ds) – DP] / (dw-ds)


FEED WATER FLOW CONTROL

The flow of water into the drum is controlled either by throttling the control valve or by varying the discharge pressure of Boiler Feed Pump through scoop control and it is ensured that true drum level (density compensated drum level) is maintained at a constant level, i.e., the set value :

a) FEED WATER CONTROL BY FEED WATER CONTROL VALVE.

There are two types of Feed Water Control Valves –1. Low Load Control Valve : Upto 20 to 30 % of load , drum level is controlled through this valve.2. Full Load Control Valve : From 20-30 % to 100 % of load, drum level is controlled through this

valve .During low load / unit start up, the control is effected through 1-Element Control system, and from 20-30 % to 100 % of load , level is controlled through 3 - Element Control system. The three Elements are : 1. Drum Level , 2. Feed Water Flow, and 3. Steam Flow.

The reason for this division of control is that below 20 % of load , one can not have reliable and accurate measurement of Feed Water and Steam Flow due to their being square root function of DP across the flow elements.

* 1-E CONTROL

Density compensated Drum Level signal is compared with the fixed set value and the resultant error signal is applied to a PID Controller, the output of which is rooted through suitable interface and control is effected through low load control valve during low load operation of the unit.

SET VALUE DENSITY COMPENSATED DRUM LEVEL

1-E CONTROL DEMAND FOR LOW LOAD CONTROL

NTPC Dadri - 6 -

PID


* 3-ELEMENT CONTROL

The most common control scheme is described here. Similar to 1-E control , Density compensated Drum Level signal is compared with the fixed set value and the resultant error signal is applied to a PID Controller (Drum level Controller), the output of which is added with pressure & temp. compensated Steam Flow signal to form a variable set point for feed water control. This signal is compared with pressure & temp. compensated Feed Water Flow and the error signal is applied to a PI + P Controller (Feed Water Controller), the output of which is rooted through suitable interface and control is effected through full load control valve during 20-100 % load operation of the unit. SET VALUE DENSITY COMPENSATED

DRUM LEVEL

PR. & TEMP. COMENSATED STEAM FLOW + + - PR. & TEMP. COMENSATED FW FLOW

3-E CONTROLBY BFP SCOOP 3-E CONTROL DEMAND

BFP PROTECTION SIGNAL

TO FULL LOAD CONTROL VALVE

NTPC Dadri - 7 -

PID

PI

MIN

SUM

PI


b) FEED WATER CONTROL BY BFP SCOOP

In order to have flow into the drum, sufficient Feed Water Pr. has to be maintained at the discharge of the Boiler Feed Pump. This discharge pressure of BFP is controlled by BFP Scoop in two ways as described below :

* DP CONTROL ACROSS F W C V

In order to have rangibility and linear operation of 3-E Drum Level Control through FW Control Valve, there must be a constant differential pressure across the FW CV. This task is executed by this control through BFP Scoop.

Differential Pressure across FW CV is measured and compared with a set point , which can be fixed or load dependent. The error signal is applied to a PI + P controller, which behaves as a master controller. For advance action Feed Forward inputs from control valve position are applied through the proportional channel of the controller . Depending upon the number of BFPs in operation , the gain of the controller is adjusted through a Automatic Gain Control circuit (AGC) . The output of this master controller is distributed to individual BFP scoop controls, which is a position controller with PI control structure. For load sharing between individual BFPs Bias stations are provided for each BFP scoop controller.

DIFF. PR. ACROSS FWCV

SET VALUE FW CV POSITION + -- Z

BFP-A in Auto BFP-B in Auto BFP-C in Auto

3-E CONTROL DEMAND BFP PROTECTION DEMAND

BIAS BIAS BIAS

F.B. F.B. F.B. - + + - + + - + +

BFP-A SCOOP BFP-B SCOOP BFP-C SCOOP CONTROL CONTROL CONTROL

NTPC Dadri - 8 -

PI+P

PI

PI

PI

MIN

+

AGC


* 3-ELEMENT CONTROL

As already indicated , Drum Level can be directly controlled by controlling BFP Disch. Pressure through BFP scoop . This control provides following advantages over the usual control through Feed Water Control Valve :

1. Wear & Tear of the FW CV is minimized , as the valve is kept fully open , providing a little restriction to the flow of Feed Water to the Drum .

2. Significant saving in electrical energy is achieved by reducing the load on BFP, as the BFP has to maintain a lower disch. pressure for maintaining proper Feed Water flow to Drum in order to maintain the level of the drum .

This control is particularly useful at higher load ( > 60% of load ) , as the control minimizes the possibility for the BFP to run at maximum discharge pressure , which is highly unsafe for BFP’s life.

To achieve the task , 3-E Control Demand signal is passed on to BFP scoop control through a minimum selection . The other signal being the control demand from Diff. Pr. Across FW CV Master. The FW CV is gradually opened to 100% by providing additional bias. The opening of the FW CV ensures that only 3-E Control Demand is passing on to the BFP scoop control .

NTPC Dadri - 9 -


BFP PROTECTION

In order to protect the BFPs from any possible damage , the pump protection circuit is implemented . Each pump has a particular characteristics ( Header Pr. vs. Flow ) for each running speed , along which the operating point lies at any given time .

For the operation of the BFPs , the manufacturers also provide a limiting curve, below which the operation of the BFP is not safe. Similarly , the discharge pressure must not more than the max. and less than the minimum safe value.

Pmax -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - COMPUTED SAFETY AREA

LIMITING CURVE

Po----------------------------------------------------------------|------BFP DISCH. | OPERATINGHEADER | POINTPRESSURE | N3

Pmin | N2 N1 | SPEED

| CURVES | ACTUAL

| LIMITING CURVE |Mo

BFP SUCTION FLOW

To achieve the task a function generator is used to compute the limiting flow as a function of the header pressure based on the limiting curve provided by the manufacturers. Actual FW flow through the BFP is compared with the above computed flow, and similarly , header pr. is compared with the max. permissible pressure . Both the errors are passed on through minimum selector and applied to limiting controller . The output of the limiting controller along with 3-E Control Demand and Diff. Pr. across FW CV Master demand is further passed through a minimum selector & applied to BFP scoop control . The pump protection acts to modify the speed of pumps in order to drive the operating point into the safe region. In addition to the above , the header pressure is also compared with minimum permissible pressure , and the error signal along with flow error signal ( i.e., error between computed and actual flow ) is passed on through a minimum selector and applied to another limiting controller, whose output and 3-E Control demand is passed on through another minimum selector and applied to full load FW valve controller .

NTPC Dadri - 10 -

FORBIDDEN ZONE


Thus, in case of activation of the pump protection circuit, ( drifting of the operating point towards forbidden zone ) , the full load control valve is also throttled ( driven in the direction of closing ) in addition to the speed of the pumps being changed to ensure a quick pressure increase after the BFPs. Both actions are complimentary ( cumulative ) and thus a quick response is guaranteed, which is very necessary to protect the Boiler Feed Pumps.

BFP DISCH. HDR. BFP SUCTION FLOW PRESSURE

Pmax Pmin A B C

SAFETYAREA

TRACK TRACK

BFP SCOOP FULL LOAD VALVE CONTROL CONTROL

BFP PROTECTION CONTROL SCHEME

NTPC Dadri - 11 -

MAX F(x)

SUM

PI

PI

SUM SUM

SUM

MIN MIN


BFP RECIRCUALTION CONTROL

In order to avoid the risk of BFPs to run dry in case of closing of the Feed Water Control Valve , the re-circulation control is fore seen . The set point is computed as a function of the pump speed or scoop position through a function generator, and is compared with the BFP suction flow . The error signal is applied to a PI controller , whose output regulates are-circulation valve in order to maintain a minimum FW flow through BFP.

SET VALUE BFP SUCTION FLOW

RECICULATION CONTROL DEMAND FOR LOW LOAD CONTROL

NTPC Dadri - 12 -

PI

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Drum Lvl Control by Pkgangwar