2000 Hook-up Book

Temperature Control Valves for Steam Service

Pneumatically Operated Valves.

Installation of Temperature Control Valves The operation and longevity of these valves depends greatly on the quality of the steam which is fed to them. The components of a tem perature control valve station are same as for a pressure reducing valve, see page 19. In addition, attention must be paid to the loca tion of the temperature sensing bulb. It should be completely immersed in the fluid being sensed, with good flow around the bulb, and, if used with a well, some heatsink material in the well to dis place the air which prevents heat transfer. The capillary tubing should not be in close proximity to high or low temperatures and should not be crimped in any fashion.

Heating Liquids By Direct Steam Injection

SYSTEM DESIGN

Pneumatically operated tempera ture control valves as shown in Fig. II-21 (page 93), provide accurate control with the ability to change the setpoint remotely. A controller, through a sensor, adjusts the air signal to the valve actuator or positioner which, in turn, opens or closes the valve as needed. Industry demands for more accurate control of temper ature and computer interfacing is making the pneumatically operat ed valves grow within the marketplace.

Where noise and dilution of the product are not problems then direct steam injection can be used for heating. Steam injection utilizes all of the latent heat of the steam as well as a large portion of the sensible heat. Two meth ods, sparge pipes and steam injectors, are used to direct and mix the steam with the product. Sparge Pipe Sizing A sparge pipe is simply a perforated pipe used to mix steam with a fluid for heating. Sizing of this pipe is based on determining the required steam flow, selecting a steam pres sure within the pipe (normally less than 20 psig for non-pressurized vessels), and calculating the num ber of holes by dividing the required steam flow by the quantity of steam that will flow through each sparge hole of a specific diameter as deter mined from Fig. 42. Holes larger than 1/8" diameter are used only on relatively deep tanks where the larger steam bubbles emitted will have time to condense before breaking the liquid surface, or where the required number of 1/8" dia. holes becomes unreasonably great. The sparge holes should be drilled 30° below the horizontal spaced approximately 6" apart and one hole at the bottom to permit drainage of liquid within the pipe, see Fig. 41. The sparge pipe should extend completely across the ves sel for complete and even heating.

Figure 40 Operating Principle of Direct Operated Valves

Valve Movement

Valve Housing

Thrust Pin Movement (Movement caused by adding temp to sensor)

Thrust Pin

Add 1°F to Sensor

Sensor Bulb

Capillary

50

40

30

3/16" Dia.

30°

1/8" Dia.

20

3/32" Dia.

10

6"

Steam Flow - lbs. per hr.

Figure 42 Steam Flow through Sparge Holes 0 10 20 30 40 50 60 70 Sparge Pipe Pressure psig

Figure 41 The sparge pipe diameter can be determined using Fig. 1 (page 4), limiting the maximum velocity to 6000 ft/min. A typical installation is shown on Fig II-42 (page 105).

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