Question

Design a Process control system To maintain the Drum level in a steam boiler system. Feed water pumped into tank is used to regulate drum level Drum level range is - 10 inches to 10 inches. It is desire to maltain level at 0 (Zero) inch with high accuracy. Major disturbance in the system is frequent change in the load steam flow out from the drum) You have to select controller from the list below a) Two position differntial gap controller b) Prportional only controller c) PID controlle • Use control technique simple Feedback Group AB.C] • Feed forward control. [Group D,E,F,G] • Components available are Thermocouples TC TC transmitters (4-20 mA) Level transmitters (4-20 mA), o Flow transmitters (4-20 mA). Pressure Transmitters (4-20 mA) FCE available are Pneumatic Control Valves (3-15 psi) . Both UP and Plconvertors. • Give reason for controller you select

Answer 1

It is given that there is much fluctuation in Boiler load..Due to this fluctuation we may experience following troubles..

The range of drum level is +10 " to -10"

In steam drum fluid (steam & water) there are huge bubbles formation whose size is affected by pressure, so if a boiler experiences a sudden extra demand for steam, its pressure drops.The drop in pressure causes the steam bubbles in the boiler water to expand, and the level measurement can show an increase in level.

The false high reading makes the water level control system reduce the flow of water into the boiler. Once boiler pressure is restored the steam bubbles contract, and the measured water level drops suddenly...So in order to mitigate these & Based on above given data, we need to select 3-element drum level controller

A three-element boiler water level control system is one which typically uses the measured water level, the steam flowrate from the boiler, and the water flowrate into the boiler to regulate the flow of water into the boiler & maintains drum level accordingly...3 element drum level controller is also called feed forward control method.

For above method we suggest, infact use the PID controller for more reliability of operation & has following main advantages

PID-Proportional integral & derivative controller..Used for continuous application like drum level controller..

1-It continuosly calculate the errors value as the difference between a desired set point (SP) and easured process variable (PV) & applies a correction based on proportional, integral & derivative terms.

2-Higher accuracy

3-No need of frequent tuning, if required can be tuned easily without much skill

4-Simple in design

5-Less expensiove

The detailed Block diagram & PID diagram is enclosed herewith for clear idea & unders standing

Drum lehel main Steam Steam flow Feed water Tomp pressure Hotel Flow Pressun Terup FWF compostated Sk compensated SP PID PV PID Single element Conta Do 3 element control

Drum IFER steam • (Thermocouples steam Jul Water with steam bubble TTC Feed control valre Positiones Ilp-converter FEG flow elementos for steam FE-I- Flow elemut-I for feed wates LT- Level transmitte. LC- teral controller FW- Feed water controle of valve-preces alce type PT- pressure transmitter 20 TT - Temperature transmitter / та - Thtroopu