Jones D.S.J., Pujado P.R. Handbook of Petroleum Processing

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SUPPORT SYSTEMS COMMON TO MOST REFINERIES 523

Protecting equipment from damage caused by ﬂow failures

Protecting downstream processes from ﬂuctuating ﬂows which could cause poor

process performance

Protecting downstream processing from ﬂuctuations in feed composition or tem-

perature

Protecting equipment from damage due to coolant failure

Types of surge volumes

There are basically two types of Surge Volumes. These are:

1. Upstream protection surge. This is a surge volume provided to protect the upstream

equipment and its associated pump from feed failure.

2. Downstream ﬂow surge. This is surge volume provided to protect downstream

equipment from feed failures or ﬂuctuations.

Examples of surge types

Process feed. Feed to process units is almost invariably on ﬂow control. Many units

also have a feed surge drum, particularly those units that are sensitive to ﬂow ﬂuctua-

tions or where complete ﬂow failure can cause equipment damage. This is an example

of “downstream ﬂow surge”. The surge volume of the drum will depend on

Source and reliability of source

Type of control at source

Variations and ﬂuctuations in source rate

Column feed from an upstream column. This feed stream will usually be controlled by

the level in the source column, hence it will be ﬂuctuating. If surge volume is provided

only in the source column it must be sufﬁcient to cater for “upstream protection”and

“downstream protection”. The use of a surge vessel would be recommended for this

case.

Feed to ﬁred heaters or boilers. The failure of ﬂow through the tubes of ﬁred heaters

or steam boilers can cause serious damage through overheating of the tubes. Conse-

quently “downstream protection”is required in this case. Invariably ﬂows to heaters

are on ﬂow control.

Reﬂux drums

1.0 When the drum only furnishes reﬂux or reﬂux and product to storage all that is

needed in terms of surge volume is sufﬁcient to provide “upstream protection”.

That is the surge volume required is only to protect the reﬂux pump from losing

524 CHAPTER 13

suction in the case of column feed failure. The pump will be required to circulate

reﬂux and cool the column down during an orderly shut down period.

2.0 When the reﬂux drum furnishes reﬂux and the feed stream to another unit then

the drum must furnish “upstream protection”surge and “downstream protection”

surge.

3.0 If there is a vapor product from the drum additional volume must be provided

in the drum to allow vapor/liquid disengaging. This will be such as to retain the

same liquid surge capacity as described above.

4.0 Should the vapor phase from the reﬂux drum be routed to the suction of a com-

pressor an even larger volume reﬂux drum will be required. This is to ensure

complete disengaging of the vapor/liquid. Internal bafﬂes or screens are also

used in the drums vapor outlet section to ensure complete phase separation.

Quantity of surge volume

The amount of surge volume will vary with the various types given above and with

the speciﬁc case in question. Sometimes this amount is set by company speciﬁcations

or, in the case of engineering contractors, by the client. Generally however the process

engineer will be responsible for setting a safe and economic surge volume. In doing

this the engineer needs to analyze each case in terms of why the surge volume is being

provided then deciding how much based on this answer. Figure 13.1 provides some

guidelines to the amount of surge that should be applied.

Some useful equations used in setting and handling surge volumes.

For surge volume vol cuft = (GPM) (minutes)/7.48

For vessel size:

Diam, D =

√

(cuft/2.35) @ L/D = 3

Diam, D =

√

(cuft/1.96) @ L/D = 2.5

Diam, D =

√

(cuft/1.57) @ L/D = 2.0

For line size:

Diam, D =

√

(GPM/25)@velocity = 10 ft/sec

Diam, D =

√

(GPM/17)@velocity = 7 ft/sec.

Diam, D =

√

(GPM/12)@velocity = 5 ft/sec.

For ﬂow rate:

ft/sec = GPM/450

For approx control valve size:

Figure 13.1.

Typical surge requirements.

Figure 13.1. (Cont.)

SUPPORT SYSTEMS COMMON TO MOST REFINERIES 527

One size smaller than line size. Thus:

Line CV



Level control

Surge volumes are maintained by controlling the amount of liquid entering or leaving

the vessel in question. There are several means of accomplishing this and these are

described and discussed in the following paragraphs:

(A) Control the surge liquid outlet on level control. This will give a close level control

but a ﬂuctuating outlet ﬂow. The level control valve (LCV) will close completely at

LLL. Thus ﬂow through the outlet line will be completely shut off at LLL.

(B) Control the surge liquid outlet on ﬂow control and provide a low level alarm.

This will give a ﬂuctuating level but will eliminate ﬂow ﬂuctuations. As there is no

LCV to restrict ﬂow at LLL then operators must physically reset the ﬂow controller to

maintain the surge volume. This has the disadvantage that the alarm condition could

be missed or even ignored, resulting in possible damage to downstream equipment,

and, in extreme cases, result in a ﬁre or explosion hazard.

give a wandering level but a smooth outlet ﬂow. The LCV reset can still however cut

off the ﬂow completely on LLL.

(D) Control the outlet ﬂow by ﬂow control and the system feed by surge volume level.

This will give close level control and also close outlet ﬂow control. The outlet ﬂow

also will not be closed off by a LLL of the surge. In the case of the feed being that

to a fractionating tower level control on the feed stream could cause tower upset

conditions. This would be particularly undesirable on fractionators that operate close

to critical conditions such as de ethanizer.

(E) Control the surge liquid on level control to an intermediate surge vessel. The

liquid from the surge vessel may then be ﬂow controlled. This is the ideal method for

controlling feed to a ﬁred heater. The only question in this case is one of economics.

Level control range

Should the decision now be to use an LC on the surge outlet (as in ‘A’or ‘E’above)

the range of the instrument needs to be determined. The range is the vertical distance

528 CHAPTER 13

between the HLL and the LLL. Now if the liquid outlet is feeding another unit which

requires a smooth ﬂow it is possible to achieve this by using a wide proportional band

and a large range. The larger the instrument range and the wider the proportional band

the less sensitive is the level controller. Consequently the ﬂow becomes smoother.

However the larger the range, the more expensive is the controller and of course the

larger is the tower or vessel in order to accommodate the greater distance between

HLL and LLL.

The selection of level control system and the level control range depends therefore on:

1.0 How many outlet streams are there from the surge vessel.

2.0 Which streams cannot tolerate complete shut off before all the available surge is

used.

3.0 The degree to which the outlet stream requires smooth ﬂow.

Pressure control for gases

Pressure control for gases is similar to level control for liquids in that it is really

a material balance control. If gas production rate varies, which is the usual case, a

pressure controller will relieve the system of gas by holding a given pressure in a

drum or tower. In this case the entire space above the liquid level actually constitutes

a surge volume.

Summary

There are two very general rules to follow in selecting a control system. These are:

If it is permissible for the product outﬂow rate to vary, use level control and a

relatively small amount of surge capacity.

If the product goes to a subsequent process where feed rate must be held constant,

use ﬂow control and considerably more surge.

The control valve

Control valves are used throughout the process industry to motivate the control of the

operating parameters listed above.

Figure 13.2 shows a conventional control valve which in this case is taken as a double

seated plug type valve. Like most control valves it is operated pneumatically by an

air stream exerting a pressure on a diaphragm which in turn allows the movement of

a spring loaded valve stem. One or two plugs (the diagram shows two) are attached to

the bottom of the valve stem which, when closed, ﬁt into valve seats thus providing

SUPPORT SYSTEMS COMMON TO MOST REFINERIES 529

1 INNER VALVE

2 BODY

3 BONNET

4 PACKING

5 STUFFING BOX

6 GLAND

7 VALVE STEM

8 YOKE

9 SPRING BARREL

1 INNER VALVE

10 SPRINGS

11 DIAPHRAM AND PLATE

12 DIAPHRAM DOME

Figure 13.2. A conventional control valve.

530 CHAPTER 13

Figure 13.3. The venturi and butterﬂy control valves.

tight shut off. The progressive opening and closing of the plugs on the valve seats

due to the movement of the stem, determines the amount of the controlled ﬂuid

ﬂowing across the valve. The pressure of the air to the diaphragm controlling the

stem movement is varied by a control parameter, such as a temperature measurement,

or ﬂow measurement, or the like. In many of the more modern reﬁneries many control

valves are operated electronically.

Figure 13.3 shows two other types of control valves in common service in the industry.

These are the venturi type and the butterﬂy type. Both these types when pneumatically

operated (which they usually are) work in the same way as described above for a plug

type valve. The major difference in these two types are in the valve system itself. In

the case of the venturi the ﬂuid being controlled is subjected to a 90

◦

angle change

in direction within the valve body. The inlet and outlet dimensions are also different,

with the inlet having the larger diameter. The valve itself is plug type but seats in the

bend of the valve body. Venturi type or angle valves are used in cases where there

exists a high-pressure differential between the ﬂuid at the inlet side of the valve and

that required at the outlet side.

Butterﬂy valves operate at very low-pressure drop across them. They can operate quite

effectively at only inches of water gauge pressure drop, and where the operation of the

conventional plug type valves would be unstable. The action of this valve is by means

of a ﬂap in the process line. The movement of this ﬂap from open to shut is made by

a valve stem outside the body itself. This stem movement, as in the case of the other

pneumatically operated valves, is provided by air and spring loads onto the stem from

a diaphragm chamber. The only major disadvantage in this type of valve is the fact

that very tight shut off is difﬁcult to obtain due to the ﬂap type action of the valve.

SUPPORT SYSTEMS COMMON TO MOST REFINERIES 531

The plug valve

There are two types of plug valves used for the conventional control valve function.

These are:

Single seated valves

Double seated valves

Single seated valves are inherently unbalanced so that the pressure drop across the

plug affects the force required to operate the valve. Double seated valves are inherently

balanced valves and are the ﬁrst choice in most services.

The conventional control valves predominantly in use can have either an equal percent-

age, or linear characteristics. The difference between these two is given in Figure 13.4.

With an equal percentage characteristic, equal incremental changes in valve stem lift

result in equal percentage changes in the ﬂow rate. For example if the lift were to

Figure 13.4. Control valve characteristics.

532 CHAPTER 13

change from 20% to 30% of maximum lift the ﬂow at 30% would be about 50% more

than the ﬂow at 20%. Likewise, if the lift increases from 40% to 50% the ﬂow at 50%

would be about 50% more than at 40%.

With a linear valve having a constant pressure drop across it, equal incremental

changes in stem lift results in equal incremental changes in ﬂow rate. For example,

if the lift increases from 40% to 50% of maximum, the ﬂow rate changes from 40%

to 50% of maximum. Thus equal percentage is the more desirable characteristic for

most applications and is the one most widely used.

Pressure drop across control valves

In sizing or specifying the duty of the control valve the pressure drop across the

control valve must be determined for the design or maximum ﬂow rate. In addition,

if it is known that a valve must operate at a ﬂow rate considerably lower than the

maximum rate, the pressure drop at this lower ﬂow rate must also be calculated. This

will be required to establish the range ability of the valve.

As a general rule of thumb the sum of following pressure drops at maximum ﬂow

may be used for this purpose:

(a) 20% of the friction drop in the circuit

(excluding the valve).

(b) 10% of the static pressure of the vessel into which the circuit discharges up to

pressures of 200 psig, 20 psig from 200 to 400 psig, and 5% above 400 psig.

The static pressure is included to allow for possible changes in the pressure level in

the system (i.e., by changing the set point on the pressure controller on a vessel). The

percentage included for static pressure can be omitted in circuits such as recycle and

reﬂux circuits in which any change in pressure level in the receiver will be reﬂected

through the entire circuit. In some circuits the control valve will have to take a much

greater pressure drop than calculated from the percentages listed above. This occurs

in circuits where the control valve serves to bleed down ﬂuid from a high-pressure

source to a low-pressure source. Examples are pressure control valves releasing gas

from a tower or streams going out to tankage from vessels operating at high pressure.

These are the circumstances where venturi or angle valves are used, as described

earlier.

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.

A circuit generally includes all equipment between the discharge of a pump, compressor or vessel and

the next point downstream of which pressure is controlled. In most cases this latter point is a vessel.