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Fluid Level Control And Indication

There are many devices used to measure or control the level of fluids in various situations.  Some of the most common applications are:  water level in boilers, water level in cooling tower sumps, and fluid levels in process tanks.

The simplest fluid level control consists of a float and rod connected to a valve, such as the Watts 750-12.  See Figure 1.  Figure 2 is a cut-away drawing of this type of valve.  A rod with a float is connected to the arm.  Rather than bending the rod to adjust the fluid level, the arm is equipped with an adjustment screw to accomplish level control.  As the fluid level drops, the float falls, pivoting the arm that is attached to the plunger and seat assembly.  The plunger moves off the seat, allowing the fluid to flow.  As the float rises, the seat and plunger assembly moves toward the valve seat, throttling the flow until the fluid level raises the ball to the desired level and flow is shut off.  This type of valve is inherently modulating.  It varies the flow of the controlled fluid, therefore level control is reasonably accurate.

Figure 1.

Figure 2.

Figure 3.

Figure 3 shows full open flow rates for various sizes.  The pressure shown is inlet pressure and would also be a  DP since these valves are only used on open tanks or sumps and directly discharge into the tank.  They are designed to be tank wall mounted.  High inlet pressures and larger valves require larger floats and longer rods than small, low-pressure valves to be able to apply enough force to close off.  Figure 4 is a chart showing the various combinations of float balls and rods used at different pressures and sizes to achieve shut-off.

Figure 4.

A common use of this style of valve is to maintain the water level in a cooling tower sump.  Every residential evaporating plate type humidifier has a small version of this style valve to keep the humidifier’s pan full of water.

These valves are inexpensive and easy to apply.  Their major failure is leaking, due to lime or dirt build-up on the valve seat.  That is why these valves should only be applied to tanks that have an overflow that can be directed in such a way as to cause no damage should the valve leak and over-fill the tank.

Another form of liquid level float control would be the common sump pump float and switch, such as the Johnson F59.  A more sophisticated sump pump control is a control that works on pressure, such as the Robertshaw 8000 series.  As the water level rises in the sump crock, the pressure (“inches of W.C.”) increases over the switch’s diaphragm until, at some preset level, the switch makes and turns on the pump motor.  Again, at a lower level, the switch breaks and shuts off the pump.  All of these sump pump water level devices are very inexpensive.

In the commercial and industrial field, there are many variations in liquids stored, sizes, shapes of tanks, and locations.  Each system will have its own unique application for tank level control and gauging.  As needs for more accurate and reliable, instruments increased, a hydraulic system was developed.  It is completely self-contained, automatic, and highly accurate.  (See Figure 5.)

Figure 5.

A float and arm assembly is connected, by means of suitable mechanical linkage, to a balanced hydraulic system.  The slightest movement of the float is transmitted to the pointer of the remote indicator.  The indicator can be calibrated in any unit of measurement, such as pounds, liters, feet and inches, gallons, etc.  Switches can be added to the indicator to activate high and low alarms, pumps, or valves.  This system is automatically temperature compensated, and since it is float actuated, the specific gravity of the liquid is unimportant.

The maximum distance the indicator can be from the tank is 250 feet.  The usual design of the float and arm is such that the leverage is in excess of that required to actuate the hydraulic system as long as the hydraulic lines are 250 feet long or less.  The hydraulic transmission lines are copper capillary tube and cannot be altered in the field.

As with any capillary tube, special care must be exercised on installation.  The tubing should be protected, and where bends are required, they should be long sweeps.  Any kinking of the capillary tubes will result in inaccurate indicator readings or no readings at all.  A properly installed system will last a very long time.  Eventually, the linkage will wear and become “sloppy”, resulting in inaccurate indicator readings.  Depending on how often the tank level is lowered and refilled will determine how long the linkage will remain within reasonable wear tolerances for accurate readings.  Therefore, these systems lend themselves to tanks where the fluid level changes slowly, such as a fuel oil tank for oil burners.

Pneumatic hydrostatic tank gauging actuation systems operate on the principle that the pressure at the bottom of a tank varies with the liquid level.  The pressure balance equal to the liquid height is converted into tank contents and indicated on a calibrated dial.  These systems are designed to measure liquids having a constant specific gravity in vented tanks.  All that needs to go into the tank is a simple bubble pipe.  There are two types of systems available.  For indication only, a system with a hand pump built into the gauge case is all that is needed, as long as constant read-out is not necessary.  (See Figure 5.)  A few strokes of the hand pump are all that is needed to obtain the pressure balance for a reading.  Hand pump systems can operate up to 150 feet from the tank.  The tubing from the bubble pipe to the hand pump is 1/4 O.D. copper tube.  Unlike capillary tubes, this tube can be field cut or spliced as needed.  During installation, care should be taken to not kink the tube.  Bends should be made with a tube bender, or carefully formed sweeps made.

Figure 6 shows the system with the hand pump eliminated and a constant source of air connected.  With a constant source of low-pressure air available, the 1/4 O.D. copper tube can be extended up to 1000 feet between the tank and indicator.  Since the indicator is now constant read, switches can be added to the indicator.

Figure 6.

Modern technology has resulted in submersible level transmitters.  These transmitters can be applied to tanks, ponds, rivers, lakes, wells, or virtually anywhere a fluid level needs to be monitored.  The transmitter measures the level of a liquid by continuously measuring hydrostatic pressure on a diaphragm seal connected to semiconductor chips.  The pressure is converted to 4-20mA outputs.  These transmitters have excellent linearity, repeatability, low hysteresis, and long-term stability — and they are very expensive!  Where high accuracy of a gauging or control system is needed, these transmitters would be a necessity.  The transmitters can be connected to any device that accepts 4-20mA, such as a UDC, DR 4500 recorder, etc., to accomplish indication/actuation as desired.

One of the world’s best known and largest manufacturers of liquid level controls is McDonnell & Miller.  They are often referred to as “The First Name” in boiler controls.  The types of boiler controls they make are essentially water level float controls.

The McDonnell & Miller 27W, 18, 18SS, and 518 are specialized versions of the Figure 1 type of valve, the 750 watts.  Many 150’s are not just controlling boiler feed pumps, but are on tanks of all sorts, operating pumps, motors, valves, etc.  The point is to not think of McDonnell & Miller controls for boilers only.

McDonnell & Miller probe controls can also be used on non-boiler applications.  See Figure 7 as an example.  Probe controls work by passing a current through the liquid to ground when the probe is in contact with the liquid.  Think of the liquid as a switch that turns “on” when the liquid is in contact with a probe, and turns “off” when not in contact with a probe.  This on/off switching is transmitted to relays to turn on or off loads such as solenoid valves, pumps, lights, etc.  McDonnell & Miller vertical probe models, such as the PCH relays with RS heads and probes, can be especially useful, lending it to many applications.

Figure 7.

Since probe controls need a current path through the liquid to ground, the liquid must be conductive.  Water and most other common liquids are conductive. 

Warning:   Probe controls should never be used with flammable liquids!   

Note:   If the tank is a non-metallic tank, such as fiberglass or concrete, a ground must be provided in the tank for the probe control to function.  This could be an additional probe, the longest probe, or a copper wire running into the tank to the bottom of the tank.

Note:   If the tank has violent turbulence in it that may cause “waves” that would make the liquid alternate rapidly between contacting or not being in contact with the probes, consider constructing a standpipe on the side of the tank for the probe head and the probes to hang into.  This standpipe should reflect the levels of control needed and will eliminate tank turbulence from having any adverse affect on the probes. 

Care should be taken to see that probe controls with two or more probes do not touch each other.  A spacer should be used to keep the probes separated.  The control panel with the relays in it can be mounted as far as 500 feet from the probe head.  With up to five level control panels now available, many more applications should be considered for probe control.  Surprisingly, probe control systems are relatively inexpensive, especially when compared to industrial vertical float operated level controls or displacer type liquid level controls.

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