Process Controls in Petroleum Processing

Abstract

This chapter focuses on the control systems which support the refining processes. This is not a definitive work on control systems, but paints the applications of control systems in a modern refinery with a broad brush. The material in this chapter starts with a discussion of control system architecture and continues with detailed descriptions of the major types of parameters controlled: flow, temperature, pressure, level, and composition. Additional material is provided on specific, common control situations. There is a brief discussion of control theory and there are procedures provided for sizing control valves. Numerous references are available for further information.

Steven A. Treese is retired from Phillips 66.

Appendix 1: Control Valve Sizing

Process Flow Coefficient (C_v) and Valve Sizing

Process flow coefficient C_V is defined as the water flow in GPM through a given restriction for 1 psi pressure drop. These C_Vs can be determined by the following equations:

$$ {C}_{\mathrm{v}}={Q}_{\mathrm{L}}\sqrt{\frac{G_{\mathrm{L}}}{\Delta P}}\kern0.3em \mathrm{f}\mathrm{o}\mathrm{r}\kern0.5em \mathrm{liquid} $$

(16)

$$ {C}_{\mathrm{v}}=\frac{Q_{\mathrm{s}}}{82}\sqrt{\frac{T}{\Delta P-{P}_2}}\kern0.3em \mathrm{f}\mathrm{o}\mathrm{r}\kern0.5em \mathrm{steam} $$

(17)

$$ {C}_{\mathrm{v}}=\frac{Q_G}{1360}\sqrt{\frac{\mu_2ST}{\Delta P-{P}_2}}\kern0.3em \mathrm{f}\mathrm{o}\mathrm{r}\kern0.5em \mathrm{gases} $$

(18)

where:

• C _v = flow coefficient

• Q _L = liquid flow in GPM at conditions

• ΔP = pressure drop across valve, psi

• G _L = specific gravity of liquid at conditions

• Q _S = steam rate in lbs/h

• P ₂ = pressure downstream of valve psia

• Q _G = gas flow in SCFH (60 °F, 14.7 psia)

• T = temperature of gas °R(°F + 460)

• S = mol weight of gas divided by 29

• μ ₂ = compressibility factor at downstream conditions

The following are some special considerations that may have to be made in determining process C_V values.

Pressure Drop

For compressible fluids the maximum usable pressure drop in equations (b) and (c) is the critical value. As a rule of thumb and for design purposes, this value is 50 % of the absolute upstream pressure. (The valve can take more than the critical pressure drop, but any pressure drop over the critical takes the form of exit losses.)

Flashing Liquids

In the absence of accurate information, it is recommended that for flashing service the valve body be specified as one nominal size larger than the valve port.

Two-Phase Flow

If two-phase flow exists upstream of the control valve, experience has shown that for fluids below their critical point, a sufficiently accurate process C _V value can be arrived at by adding the process C _V values for the gas and liquid portions of the stream. The calculation is based on the quantities of gas and liquid at upstream conditions. The valve body is specified to be one nominal size larger than the port to allow for expansion.

Valve Rangeability

The rangeability of a control valve is the ratio of the flow coefficient at the maximum flow rate to the flow coefficient at the minimum flow rate (R = C _V Max/C _V Min). Valve rangeability is actually a criterion which is used to judge whether a given valve will be in a controlling position throughout its required range of operation (neither wide open nor fully closed). In practice the selection of the actual valve to be installed is the responsibility of the instrument engineer. As the process engineer is usually the person responsible for the correct operation of the process itself; however, he must be satisfied that the item selected meets the control criteria required. He must therefore satisfy himself that the valve will control over the range of the process flow.

Control valves are usually limited to a rangeability of 10:1. If R is greater than 10:1, then a dual-valve installation should be considered in order to assure good control at the maximum and minimum flow conditions.

In some applications, particularly on compressor or blower suction, butterfly valves have been specified to be line size without considering that as a result the valve may operate almost closed for long periods of time. Under this condition, there have been cases of erosion resulting from this. It is recommended therefore that butterfly valves be sized so that they will not operate below 10 % open for any appreciable period of time and not arbitrarily be made line size.

Valve Flow Coefficient (C_v′)

In order to ensure that the valve is in a controlling position at the maximum flow rate, the valve C _V′ is the maximum process C _V value determined above, divided by 0.8. The reasons for using this factor are that:

• It is not desirable to have the valve fully open at maximum flow since it is not then in a controlling position.

• The valves supplied by a single manufacturer often vary as much as 10–20 % in C _V.

• Allowance must be made for pressure drop, flow rate, etc., values which differ from design.

Control Valve Sizing

Control valve sizes are determined by the manufacturers from the process data submitted to them. However, there are available some simple equations to give a good estimate of the required valve sizes to meet a process duty. Three of these are given below: Single-seated control valve sizes may be estimated by:

$$ S\left(\mathrm{inches}\right)={\left[\frac{\mathrm{V}\mathrm{alve}\;{C}_{\mathrm{V}}}{9}\right]}^{1/2} $$

(19)

Double-seated control valve sizes may be estimated by:

$$ S\left(\mathrm{inches}\right)={\left[\frac{\mathrm{V}\mathrm{alve}\;{C}_{\mathrm{V}}}{12}\right]}^{1/2} $$

(20)

Butterfly valve sizes may be estimated by:

$$ S\left(\mathrm{inches}\right)={\left[\frac{\mathrm{V}\mathrm{alve}\;{C}_{\mathrm{V}}}{20}\right]}^{1/2} $$

(21)

The constants (9, 12, or 20) in the denominators of these equations can vary as much as 25 % depending on the valve manufacturer.

A control valve should be no larger than the line size. A control valve size that is calculated to be greater than line size should be carefully checked together with the calculation used for determining line size. Usually, a control valve size should be one size smaller than line size.

Once the valve size is estimated and the valve C _V known, then the percent opening of the valve at minimum flow and maximum flow can be obtained by dividing the respective process C _V values conditions by the selected valve C _V. This information is normally required to check the percent opening of a butterfly at minimum flow. It is not normally necessary to calculate it for any other type of valve.

Valve Action on Air Failure

In the analysis of the design and operation of any process or utility system, the question always arises on the action of control valves in the system on instrument air failure. Should the control valve fail open or closed is the judgment decision of the process engineer after evaluating all aspects of safety and damage in each event. For example, control valves on fired heater tube inlets should always fail open to prevent damage to the tubes through low or no flow through them when they are hot. On the other hand, control valves controlling fuel to the heaters should fail closed on air failure to avoid overheating of the heater during the air failure.

The failure action of the valve is established by introducing the motive air to either above the diaphragm for a failed open requirement or below the diaphragm for a failed shut situation. The air failure to the valve above the diaphragm allows the spring to pull up the plugs from the valve seats. Air failure to valves below the diaphragm forces the spring to seat the valves in the closed position. Failure of a valve in place or locked is also possible, but seldom used.