Purpose of 100% and 30% Feed Control Valves in a Power Plant

In this topic, we will discuss the purpose of 100% and 30% valves. Why are both these valves required in power plants?

Before starting with the discussion will see some basic aspects

What is a Control Valve?

A control valve is installed in a pipeline to regulate the flow rate of the fluid (Liquid, Gas, or Steam) as a response to the controller output (manipulated variable).

The controller can adjust the valve opening or closing at any stage between 0% to 100%.

The control valve in a pipeline does the following tasks.

  • Stopping and starting of flow
  • Reduce and raise the flow rate
  • Controls the flow direction

What is Non-Return Valve (NRV)?

This is a one-way valve that allows the fluid to flow in one direction only.

As its name implies, non-return means there is only one-directional flow. This device ensures there is no backflow of fluid in the pipeline.

These valves are generally used in pump discharges and steam water pipelines.

The direction symbol provided on the valve shows the direction of the fluid flow.

What is a Boiler Feed Pump?

 The Boiler Feed Pump is a pump that supplies De-aerated and LP-dosed Feed water to the boiler Drum against boiler drum pressure.

The discharge pressure of the pump must be more than the drum. The feed pumps are kept in hot Standby if one pump trips then the other standby pump starts automatically.

In a Power Plant, the feedwater system includes De-Aerator, Feed Pump, Feed control Station, HP heaters, economizer, and Boiler drum.

Boiler Feed Pump

In a power plant, the 100% valves and 30 % valves are known as feedwater control valves that are installed in feed water control stations. The feedwater control valves are the most important instruments in a water-steam circuit.

This Feed Water control station consists of one 100% Feed Control Valve (FCV-301), and one 30% Feed Control Valve (FCV-302). And the bypass line (without a control valve) is used to feed water to the boiler during Control Valve maintenance.

There is an isolation valve provided in the pipeline before and after the control valves. The bypass line size is the same as the mainline to ensure the same flow rate.

The control valve used is a motor-operated valve with a Non-Return Valve (NRV) that restricts the backflow of feed water.

Depending upon the water requirement in the boiler, these valves are used.

100% valve is used for main feedwater control with boiler working at full load at the rated design parameter.

30% Valve is used during the start-up of the boiler and during Hydraulic tests.

100% Feed Control Valve

Boiler Feed Water Control Valves
Figure 100 % Control Valve : (FCV-301 ON)

The main objective of the feedwater control station is to balance the Main Steam Flow and Feedwater flow in proportion to maintain a safe water level in the steam drum at any boiler load condition.

The Steam flow in TPH must be Equal to Feedwater Flow in TPH.

Steam Flow = Feedwater Flow

 The boiler Drum Level control is a typical 3 element controller on a full load of 120 TPH.

The three-element controls are

  1. Drum level
  2. Main steam flow
  3. Feedwater flow

During normal operation, when the boiler is working in full load using 3-element control, the feed water to the boiler is supplied with 100% BFP and 100% Feed Control Valve.

When the 100% control valve is working, the bypass line is kept closed by closing the Upstream and Downstream isolating valve. Usually, the pressure of the feed water to the boiler must be 15% to 20% higher than that of the boiler operating Pressure 

If the boiler operating pressure is 110 Kg/cm2 then the feed water pressure must be 135 to 140 Kg/cm2   (i.e. 15% to 20% higher ).

Feedwater Pressure > Drum Pressure > Main Steam Pressure

30% Feed Control Valve

30% Feed Control Valve
Figure: 30% Control Valve: (FCV-302 ON)

During Start-Up of the Boiler

The feed water start-up valve controller represents the single Element (1-E) portion of the feedwater control loop that is used during the start-up of the boiler.

When the boiler generates a steam load of 30% the feedwater control is switched to 3-Element mode. During this time, this 30% valve is kept closed.

During Hydraulic Test

The boiler needs to be shut down for maintenance and repair for some period. And gets started up after maintenance is over.

Before the Start-up of the boiler, the hydraulic test is the most essential test that is needed to be carried out to check the leakages at all the pressure parts of the boiler and its axillaries. For this purpose, only 30% Boiler Feed Pump And 30% Feed Control valve is taken into line. 100% Feed Control valve is not used at this time.

As a start-up, feedwater control valves are subjected to high-pressure drop because the pressure required is less and low flow requirement during the initial phases of the boiler start-up operation.

When the Boiler load picks up above 30% of working Pressure, the feedwater control station switches to 100%. (FCV-301) as these 100% feedwater control valves are subjected to lower pressure drop and higher flow requirements.

This 30% Feed Control valve is kept closed, and 100 % Feed Control valve is taken into line.

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