However, in solids applications, material entering and leaving the vessel affect the solid/gas interface. As such, this interface is typically not horizontal, so a single level measurement may not be representative of the amount of material in the vessel. In these applications, the level measurement reflects the level at one point in the vessel. Level measurements can change rapidly when the material level that is sensed changes rapidly. In some applications, multiple level measurements may be needed to provide a more accurate indication of the inventory of material in the vessel.

For example, a rat-hole may form as solid material leaves the vessel. If the level measurement reflects a point in the rat-hole, the measured level will decrease (as expected). However, if the material remaining on the sides of the vessel falls and fills in the rat-hole, the level will abruptly (and unexpectedly) increase. Sensors should be located such that the measured level represents the actual level while avoiding rat-hole affects. Multiple sensors may be needed if an appropriate location cannot be found.

In addition, most non-contact level sensors cannot accurately measure distances that are close to the sensor itself. Sensors are typically installed to allow the transmitter to disregard measurements at these distances. This is often called the blanking distance.

Excerpted from The Consumer Guide to Non-Contact Level Gauges.

In one such flowmeter application, the capillary tubes were of equal length --- following the "rule of thumb" to use capillary tubes of equal length.  However, the excess capillary tubing to the closer tap (approximately 4 meters) was coiled inside the base of the transmitter enclosure.  This would not be a problem if the temperature of the capillary tubing inside the base and exposed to the closer tap were the same as the temperature as the exposed capillary tubing to the farther tap. 

In this installation, the instrument enclosure was cooled so the temperature of the capillary tubing was different.  In particular, the temperature of the capillary tubing to the closer tap was colder than that to the farther tap.  Compounding the problem, the exposed capillary tubing to the farther tap was exposed to sunlight during part of the day.  These conditions affect the measurement in the same direction so the measurement error would be greater than either individual error. 

One of the capillary tubes was coiled while the other was not.  Although it might seem that this would affect the measurement, the measurement should not be affected because the pressure increases/decreases in the coiled impulse tube effectively cancel so the static pressure at the transmitter is dependent upon the elevations of the tap and the transmitter.  

In many applications the transmitter enclosure is heated and can create similar problems.  This often occurs in colder climates where the capillary tubing to the closer tap will be warmer than the capillary tubing to the farther tap that is exposed to the cold climate.  These conditions affect the measurement in the same direction so the measurement error would be greater than either individual error --- albeit in the opposite direction as compared to an installation where the instrument enclosure is cooled. 

Remember that following a "rule of thumb" may be a good thing, but doing so without a complete understanding can create problems.

This article originally appeared in Flow Control magazine. 

1. 65 precent pressure and 50 percent energy
2. 65 precent pressure and 80 percent energy
3. 80 precent pressure and 50 percent energy
4. 80 precent pressure and 80 percent energy
   

Variable speed drives are increasingly applied in many applications to reduce energy consumption.  Reducing the speed of a centrifugal pump reduces the pump discharge pressure by the square of its speed.  Therefore, the discharge pressure will be approximately 64 percent of the full speed discharge pressure when the pump is operated at 80 percent of speed.  Answer C and Answer D are not correct. 

Reducing the speed of a centrifugal pump reduces the energy consumption by the cube of its speed.  Even a small speed reduction such as 5 percent can save almost 15 percent of the energy consumed at full speed.  In this case, operating at 80 percent speed reduces the energy consumption to approximately 51 percent of the energy required to operate the pump at full speed.  Therefore, Answer A is correct.

Additional Complicating Factors

Many variable speed drive applications and installations require attention to detail to ensure that the motor and its mechanical equipment can function properly in operation.  In addition, failure to attend to certain details of lower speed operation can damage the equipment.

This article originally appeared in Flow Control magazine.