Boiler Drum Level Control

I've been working on this post about level control in steam boilers for a while now.  It looks like today is the day I'll finally manage to get that done.

First, let's look at level control for a simple tank containing liquid - a water tank.  To make things interesting (and realistic), our tank will have an outflow demand that can change quite a bit.  Our goal is to control the tank level - we want keep the tank from going empty during high demand, and we don't want it to overflow when demand for water drops off.

One simple way to keep our tank from running empty or overflowing is to install two level switches at different levels in the tank.  These are typically float-activated electrical switches.  A switch near the bottom of the tank will turn on a make-up pump, and a switch near the top will shut the pump off again when the tank is topped off.

Below, a crude home-made MS Paint drawing of what I described above.  Obviously I will never be able to earn a living with my proficiency with MS Paint.

If you aren't used to viewing process drawings, I'll explain what the diagram represents.  The arrow to the right indicates water being drawn from the tank.  The arrow to the left is water being supplied to the tank.  The round item is a pump that provides enough pressure to fill the tank quickly, once it is turned on.  The two knobby things to the right are level switches.  When the tank level gets low, the lower level switch turns on the pump, and soon afterward, when the tank is full, the upper level switch will turn the pump off.  Pretty simple.

This arrangement works pretty well for tanks with large volumes or infrequent demand.  Every time the pump cycles on and off, the electrical contacts experience wear and tear, as do the pump and motor.  For these reasons, the use of this arrangement is somewhat limited, and definitely not to be used in a small tank with frequent changes in demand.

A similar arrangement to the above level control scheme is to have the pump run continuously.  In the case below, the pump's discharge valve will be opened and closed based on the position of float  switches.  When the water level goes low, the valve is open, and when the tank is full the valve shuts.  This arrangement works well for when water is needed elsewhere and the pump must be run continuously to supply water for other processes, in addition to filling the tank.

 In the diagram below, we move to a more practical and commonly used method for level control.  With this arrangement we throttle the input flow, and maintain a very consistent level in the tank.  The previous arrangements either had full make-up flow or no flow at all.  In this case we have a level transmitter in the tank.  This transmitter will send an output signal that will increase or decrease as the level in the tank rises and falls.  That signal will be sent to the pump discharge valve, causing it to open or close. 

 In the above drawing, the level transmitter is a "bubbler type", where a small trickle of air bubbles down through a tube and out an opening at the bottom.  The higher the level in the tank, the higher the back-pressure in the transmitter at the top.  The transmitter is simply a pressure gauge that converts this variable back-pressure to an electrical signal.  Bubbler type transmitters are used when the water has debris in it, or when the liquid is thick and would tend to foul other types of transmitters.  Sonic type level transmitters are also useful for these situations.

The transmitter is calibrated so zero is when the tank is above the outflow and 100% is when the tank is almost full.  Typically with this arrangement, the operator can enter a desired level for the valve to control at, and the system will maintain level at that value.  The units of measurement could be percentage full, or more typically displayed as units of length above the tank bottom (feet, inches, etc).

These are some of the typical schemes used to control water level in tanks.  However boilers are a different sort of animal, and I will explain why below.

Boilers cannot use a bubbler type level transmitter to determine level, because of the large pressure swings they undergo during start-up and shut down.  Within just a few hours, a boiler will go from zero pressure to thousands of PSI (pounds per square inch) of pressure.  In this case, we use a differential pressure (D/P) transmitter.    The level is determined by the pressure difference due to the weight of the liquid in the pressure vessel.  As the pressure in the vessel increases or decreases, it does so equally on the top and bottom of the vessel, so the only difference felt by the transmitter is due to the weight of the liquid, regardless of the vessel's internal pressure

Boilers, by their very nature, turn large quantities of water into steam.  This is accomplished by exposing water in vertical tubes to very high levels of heat flux.  What enters at the bottom of these vertical tubes is 100% water, but is a mixture of water and steam as it exits at the top.  Boilers have an internal circulation loop.  This design ensures that water continuously circulates in tubes exposed to the heat.  This circulation path can either use natural circulation or use forced circulation using a special pump.

Below:  A Natural Circulation boiler on the left, and Forced Circulation boiler on the right.
By Tripp193 - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=47940974

At the top of the boiler, the vertical tubes are connected to a steam drum.  The steam drum segregates the water/steam mixture exiting the tubes, allowing dry steam to be supplied from the boiler to other equipment - typically a steam turbine.  The steam drum allows the segregated water a path to circulate back to the bottom of the boiler through downcomers - paths outside of the heat flux.  The steam drum provides a point where boiler water level can be monitored and controlled - which is the point of this post.

It's critical in a pressure boiler to monitor steam drum level at all times, because incredibly bad things happen if the boiler does has too much or too little water.  If the boiler runs low on water, internal circulation will be lost.  This internal circulation is what cools the boiler tubes that are in the heat.  Without water inside them to keep them cool, they can become red-hot and burst, releasing the steam pressure in an explosive event.

Below is a CCTV clip of a small low pressure (150 psi) boiler rupturing a tube due to low water level.  If this were to occur to a power plant boiler, nothing would be left standing, including the camera.  Boiler explosions are pretty rare due to strict regulations and frequent safety testing.  Action starts at 00:25.

On the other hand, if you have too much water in the boiler, it can be carried over (a situation called "carryover" oddly enough) with the steam, and damage the steam turbine.  The steam turbine rotates at 3600 RPM (or 60 times per second), so the turbine blades have a phenomenal amount of speed and rotational energy.  When they get struck by anything other than dry steam, bad things happen.

If the steam drum is run slightly too high over a long period of time, water droplets can pass through the separators and dryers, eroding the steam turbine blades over a period of time and causing a loss of efficiency.  If the water level suddenly goes too high and a large volume of water gets to the turbine, major damage will occur immediately.  Water will hit the fast-moving blades almost as though it were a solid.  Turbine blades will be bent or knocked loose, and anything downstream will be damaged by the loose blade(s).  The turbine will also experience high axial thrust, because water has far more mass than steam, and the turbine is designed for a certain mass flow rate of steam.  This sudden large thrust frequently damages the thrust bearing as well, and clearance may be lost between stationary and rotating blades, further damaging the machine.

These are the reasons that it's important to maintain boiler drum level.

Boilers are not water tanks though - and this adds some complexity to level control.

1.  Boilers are equipped with economizers.   The economizer pre-heats boiler feedwater with waste heat in the exhaust gas to improve efficiency.   This is gas that previously boiled water and superheated steam - it is at reduced temperature, but still has a great deal of energy that can be captured by the economizer.

In the image below, feedwater enters at the bottom right, passes through the economizer, and enters the steam drum.  Ideally, it is preheated to the same temperature as the water in the steam drum.

The boiler drum level control valve can be placed either at the economizer inlet or outlet.  There are significant disadvantages with each arrangement, both of which occur during start-up.

During start-up, when boiling first occurs, drum level will increase rapidly, because steam bubbles begin lofting slugs of water up out of the furnace into the drum.   During this part of the start-up operators typically dump a great deal of water out of the drum.  Also during this period, the drum level control valve will be shut for quite a while. 

After a while, when the metal in the boiler has warmed, steam pressure will start building.  At this point, steam pressure begins to compress the steam in the furnace tubes, and the drum level situation reverses.  Suddenly operators need to stop dumping water and make up a lot of water to keep the drum level from going too low and tripping the furnace (or gas turbine).

This is the point in the start-up where the location of the drum level control valve affects boiler operation.

If the drum level control valve is at the economizer inlet, there is a strong probability that the economizer has also partially or completely steamed off into the drum, and is no longer packed with water.  Before you can add water to the steam drum, which you need immediately, you have to first re-fill the economizer.  The economizer doesn't have a level indicator, so you are on your own trying to figure out how much to open the drum level control valve to turn the dropping level around.

On the other hand, the drum level control valve can be located at the outlet of the economizer.  A different issue arises with this design, and that is over-pressurizing the economizer.  Once again, as boiling begins, and drum level increases, the level control valve remains shut for a long period of time.  In this case, water in the economizer is trapped between the closed drum level control valve and the boiler feedwater pump check valve.  This solid mass of water is being very rapidly heated, and would like to boil, but has no place to go.  So the pressure builds until it blows an economizer safety, which typically will not re-seat once the pressure is released.

From experience with both designs, I prefer the economizer outlet design, provided there is a dedicated pressure relief path for the economizer during startup.

2.  Boilers, as I discussed above, experience shrink and swell.

Shrink happens because when boiler pressure increases, it causes steam in the furnace tubes to partially collapse, and this in turn causes steam drum level to dive.

Swell is the reverse effect:  when boiler steam pressure drops, steam in the furnace tubes expands, pushing a steam/water mixture into the drum, causing water level to increase.

You get a lot of pressure transients during start-up.  When steam bypass or vent valves modulate, you get large pressure changes and large drum level swings.  When you roll the steam turbine, pressures and levels swing.  When you synchronize the steam turbine and begin loading it, you get a big pressure swing and accompanying drum level swings.

Start-up is made even more complex by the fact that typical gas turbine boilers have three steam drums and a hotwell that supplies make-up water to all of them.  All of these levels need to be properly maintained at all times, or the plant will trip.

Once the plant is up to its normal operating range, drum level control can be handled with automatic control logic.  That logic is a little different than you might expect, and that is due to the effects of shrink and swell.

Suppose there is a transient pressure drop in the system.  Drum level will go up due to swell.  The drum level control valve will cut back and reduce the amount of feedwater going to the boiler drum.  Once the pressure recovers, the drum level will shrink due to collapse of the steam in the boiler tubes, and the feedwater valve will open to compensate.  You can get into big non-dampening drum level swings simply due to changing load on the unit.

The answer to this drum level instability issue is to use "Three Element Control", and this is only necessary for systems with boiling water in them.  We measure water flowing into the boiler, steam flowing out of the boiler, and water level.  The steam flow measurement provides an anticipatory signal for the drum level control valve to adjust feedwater flow in advance of what the drum level transmitter is showing.

So, if the 3-element controller sees an upward shift in steam flow, the drum level control valve will begin coming open, even if the drum level transmitter is above the normal setpoint.  This design provides a feed-forward that more steam is leaving the boiler, so more water needs to be added, predicting in advance what will happen to boiler drum level.

Interesting stuff.