Liquid level

Level measurements are used either to control level in vessels or apparatus, or to measure throughput. Level measurements are important in order to estimate inventories and their movement, or to assure the smooth operation of processes. Fur- 

A great variety of level measurement techniques are available. These involve point-contact, visual, buoyancy, float, and hydrostatic methods, and radio-frequency, ultrasonic, microwave, nuclear radiation, resistance tape, and thermal level systems [3]. 

In the buoyancy method, displacement bodies measure levels in liquid containers. By measuring the difference in weight of a partially submerged body at various degrees of submergence, one may determine the level of the liquid in which it is 

In the hydrostatic method, the pressure at the base of the liquid, which is related to the height of the liquid above the base, is measured. The measured pressure depends directly on the liquid height according to the Bernoulli equation or mechanical energy balance [10-12] [Eq. (6), where Z2 is the level at the surface of the liquid, Zi is the level at the base, p2 is the pressure at the surface, pi is the pressure at the base, p is the liquid density, and g is the acceleration due to gravity). In pressurized reservoirs one measures the pressure difference p — P2 between the base and the gas space. 

In a variant of this method, an inert gas is bubbled through the liquid with a dip tube. If the density of the liquid is knovm, one can also determine the level through weighing. 

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In preliminary process design, the primary consideration is contact by inhalation. This happens either through accidental release of toxic material to the atmosphere or the fugitive emissions caused by slow leakage from pipe flanges, valve glands, and pump and compressor seals. Tank filling causes emissions when the rise in liquid level causes vapor in the tank to be released to the atmosphere. 

Vapor Treatment. The vapors from the tank space can be sent to a treatment system (condenser, absorption, etc.) before venting. The system shown in Fig. 9.1 uses a vacuum-pressure relief valve which allows air in from the atmosphere when the liquid level falls (Fig. 9.1a) but forces the vapor through a treatment system when the tank is filled (Fig. 9.16). If inert gas blanketing is required, because of the flammable nature of the material, then a similar system can be adopted which draws inert gas rather than air when the liquid level falls. 

In this accident, the steam was isolated from the reactor containing the unfinished batch and the agitator was switched ofiF. The steam used to heat the reactor was the exhaust from a steam turbine at 190 C but which rose to about 300°C when the plant was shutdown. The reactor walls below the liquid level fell to the same temperature as the liquid, around 160°C. The reactor walls above the liquid level remained hotter because of the high-temperature steam at shutdown (but now isolated). Heat then passed by conduction and radiation from the walls to the top layer of the stagnant liquid, which became hot enough for a runaway reaction to start (see Fig. 9.3). Once started in the upper layer, the reaction then propagated throughout the reactor. If the steam had been cooler, say, 180 C, the runaway could not have occurred. ... 

An alternative and probably now more widely used procedure is to raise the liquid level gradually until it just touches the hanging plate suspended from a balance. The increase in weight is then noted. A general equation is... 

The purification train. The oxygen is led from the cylinder through Ordinary flexible rubber condenser tubing to the constant level device A (Fig. 85). This consists of two concentric tubes (approximately 2 cm. and 0-5 cm. respectively, in diameter the inner tube being narrowed and curved at the bottom as shown) immersed in 50% aqueous potassium hydroxide contained in the outer vessel (diameter 3-5 cm.). Then by adjusting the liquid level in A the pressure of oxygen may be kept constant, and at a maximum of about... 

Apparatus. The apparatus is made of Pyrex glass, in one piece. It consists of a shaped bulb A (Fig. 89 of about 30 ml. capacity in which the reaction takes place, provided with an inclined inlet B at the side and a vertical ascension tube D. B serves not only as an inlet for the admission of the carrier gas but also as the route by which the reagents and test sample are introduced into the apparatus. B ends in a small ground-glass joint into which fits ajoint carrying a capillary-tube which projects well down into the bulb A (the end of the capillary should be just above the liquid level when the apparatus is charged for the determination). The upper extension of this capillary beyond the joint is provided with a tap C to control the rate of flow of the carrier gas. 

Measurement by Liquid Level. The flow rate of Hquids flowing in open channels is often measured by the use of weirs (see Liquid-LEVEL measurement). The most common type is the rectangular weir shown in Figure 22e. The flow rate across such a weir varies approximately with the quantity. Other shapes of weirs are also employed. Standard civil engineering handbooks describe the precautions necessary for constmcting and interpreting data from weirs. 

Closed Vessels. Liquid level can be measured by the static pressure method also at non atmospheric pressures. However, ia such cases the pressure above the Hquid must be subtracted from the total head measurement. Differential pressure measuriag instmments that measure only the difference ia pressure between the pressure tap at the bottom of the tank and the pressure ia the vapor space are used for this purpose. At each tap, the pressure detected equals the Hquid head pressure plus the vapor pressure above the Hquid. Siace the pressure above the Hquid is identical ia both cases, it cancels out. Therefore, the change ia differential pressure measured by the instmment is due only to the change ia head of Hquid ia the vessel. It is iadependent of the pressure within the tank and is an accurate measure of the level. 

Process Measurements. The most commonly measured process variables are pressures, flows, levels, and temperatures (see Flow LffiASURELffiNT Liquid-levell asurel nt PressureLffiASURELffiNT Temperaturel asurel nt). When appropriate, other physical properties, chemical properties, and chemical compositions are also measured. The selection of the proper instmmentation for a particular appHcation is dependent on factors such as the type and nature of the fluid or soHd involved relevant process conditions rangeabiHty, accuracy, and repeatabiHty requited response time installed cost and maintainabiHty and reHabiHty. Various handbooks are available that can assist in selecting sensors (qv) for particular appHcations (14—16). 

Liquid Level. The most widely used devices for measuring Hquid levels involve detecting the buoyant force on an object or the pressure differential created by the height of Hquid between two taps on the vessel. Consequently, care is required in locating the tap. Other less widely used techniques utilize concepts such as the attenuation of radiation changes in electrical properties, eg, capacitance and impedance and ultrasonic wave attenuation. 

Fig. 13. Cascade control schemes, where TC = temperature controller FC = fuel gas flow controller and LC = liquid level controller, (a) Simple circuit having no cascade control (b) the same circuit employing cascade control and (c) and (d) Hquid level control circuits with and without cascade control,...

Example 6 Losses with Fittings and Valves It is desired to calculate the liquid level in the vessel shown in Fig. 6-15 required to produce a discharge velocity of 2 m/s. The fluid is water at 20°C with p = 1,000 kg/m and i = 0.001 Pa - s, and the butterfly valve is at 6 = 10°. The pipe is 2-in Schedule 40, with an inner diameter of 0.0525 m. The pipe roughness is 0.046 mm. Assuming the flow is tiirhiilent and taking the velocity profile factor (X = 1, the engineering Bernoulli equation Eq. (6-16), written between surfaces 1 and 2, where the... 

Material and energy balances are based on the conservation law, Eq. (7-69). In the operation of liquid phase reactions at steady state, the input and output flow rates are constant so the holdup is fixed. The usual control of the discharge is on the liquid level in the tank. When the mixing is adequate, concentration and temperature are uniform, and the effluent has these same properties. The steady state material balance on a reacdant A is... 

On/Off Control An on/off controller is used for manipulated variables having only two states. They commonly control temperatures in homes, electric water-heaters and refrigerators, and pressure and liquid level in pumped storage systems. On7off control is satisfac-toiy where slow cychng is acceptable because it always leads to cycling when the load hes between the two states of the manipulated variable. The cycle will be positioned symmetrically about the set point only if the normal value of the load is equidistant between the two states of the manipulated variable. The period of the symmetrical cycle will be approximately 40, where 0 is the deadtime in the loop. If the load is not centered between the states of the manipulated variable, the period will tend to increase, and the cycle follows a sawtooth pattern. 

Three examples of simple multivariable control problems are shown in Fig. 8-40. The in-line blending system blends pure components A and B to produce a product stream with flow rate w and mass fraction of A, x. Adjusting either inlet flow rate or Wg affects both of the controlled variables andi. For the pH neutrahzation process in Figure 8-40(Z ), liquid level h and the pH of the exit stream are to be controlled by adjusting the acid and base flow rates and w>b. Each of the manipulated variables affects both of the controlled variables. Thus, both the blending system and the pH neutralization process are said to exhibit strong process interacHons. In contrast, the process interactions for the gas-liquid separator in Fig. 8-40(c) are not as strong because one manipulated variable, liquid flow rate L, has only a small and indirec t effect on one controlled variable, pressure P. 

Continuous mea.sui ements. An example of a continuous measurement is a level measurement device that determines the liquid level in a tank (in meters). 

In a process loop with a pneumatic controller and a large process time constant. Here the process time constant is dominant, and the positioner will improve the linearitv of the final control element, Some common processes with large time constants that benefit from positioner application are liquid level, temperature, large volume gas pressure, and mixing,... 

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