Pressure Measurement & Control


Digital control valve for preset loading control

January 2002 Pressure Measurement & Control

The Smith 210 Digital Control Valve, supplied by DLM in southern Africa, ensures accurate control of fluids used in the petrochemical industry. This is done electronically, and is typically used in conjunction with either the Smith AccuLoad, or Metermatic EM5-SPP electronic bay controller.

Features include:

* Simple control loop.

* Control loop isolation valves.

* Low pressure drop.

* Separate opening and closing speed control.

* Compound spring.

* Horizontal or vertical applications.

* Full range of optional control functions.

Benefits

* Compact design and mechanical simplicity.

* Drip-tight closure.

* In-line serviceability.

* Low maintenance costs.

* No external packing gland.

In operation, the Smith Model 210 valve consists of a Smith 200 Series valve with two solenoid controls (Figure 1).

Figure 1
Figure 1

The normally-open (NO) and normally-closed (NC) solenoids, located in the upstream and downstream portions of the control loop, respectively, control the operation of the valve. With both solenoids energised, high upstream pressure is blocked allowing the product in the cover to vent to low downstream pressure, opening the main valve. Conversely, de-energising both solenoids allows high upstream pressure to close the valve.

Energising just the NO solenoids locks fluid in the valve cover; this locks the valve poppet in a fixed position, to maintain a constant flow rate as long as operating conditions do not change. When operating conditions (eg pressures) change, causing a change in flow rate for that fixed valve opening, the flow controller (eg Metermatic EM5-SPP or AccuLoad) signals the appropriate solenoid to open momentarily to readjust flow rate back to its set value. When the set flow rate changes (eg from low flow start to high flow limit, or during multistep valve shutdown), the appropriate solenoid is signalled to open until flow rate adjusts to the new set value. See Figure 2 for a typical truck loading flow rate sequence.

Figure 2. Typical load cycle
Figure 2. Typical load cycle

Located between each solenoid and the main valve port is a valve response control device, typically a ball valve. This device is used to fine-tune the opening/closing rate of the valve, as well as providing total control loop isolation for ease of service. Adjustment of these devices controls the flow to the cover chamber, permitting adjustments based on product viscosities and pressures.

Other applications include:

* Electrical block valve.

* Mechanical block.

* Flow limiting.

* Pressure relief.

* Pressure sustaining.

* Differential back pressure sustaining.

* Check with thermal relief.

* Two stage electro-hydraulic set stop with flow limiting.

DLM

(011) 975 1171

wayne@dlm.co.za





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