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

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SUPPORT SYSTEMS COMMON TO MOST REFINERIES 533

Should the control valve fail open or closed is the judgment decision based principally

on 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

over heating of the heater during the air failure.

The failed 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.

Sizing a control valve

The sizing procedure for sizing control valves is given in Appendix 13.3 of this

chapter.

13.2 Offsite systems

In most reﬁneries the off sites facilities are often the major capital cost center second

only to the process plants themselves. Indeed in some instances where the off sites

include one or more complex jetties it can be the principal capital cost center. Among

the major units found in most reﬁneries are:

Storage

Product blending

Road and rail loading

Jetty facilities

Waste disposal

Efﬂuent water treating

Many reﬁneries consider that the ﬂare is part of the safety system. To a large extent

this is justiﬁable as most relief valves exhaust to the ﬂare as does the process and

other vent systems. For the purpose of this publication however the ﬂare system is

considered as part of the off sites and speciﬁcally comes under the section on waste

disposal.

Storage facilities

The crude oil feed and the processed products are stored in storage tanks of various

sizes and types. These tanks are usually collected and located together in the reﬁnery

area suitably deﬁned as ‘The Tank Farm’. There will be many other tanks (usually

534 CHAPTER 13

Figure 13.5. Cone roof tank.

much smaller than those in the tank farm) which will contain chemicals to be used in

the processes and the slops or spent chemicals from the various processes and utility

plants. Most reﬁnery storage tanks fall into the following categories:

Atmospheric storage

Pressure storage

Heated storage

Atmospheric storage

As the name implies, all atmospheric storage tanks are open to the atmosphere, or are

maintained at atmospheric pressure by a controlled vapor blanket. These tanks fall

into two categories:

Cone roof tanks

Floating roof tanks

Cone roofed tanks

Among the most common atmospheric storage tank is the cone roofed tank shown

in Figure 13.5. This tank is used for the storage of non-toxic liquids with fairly

low volatility. In its simplest form the roof of the tank will contain a vent, open to

atmosphere, which allows the tank to “breathe”when emptying and ﬁlling. A hatch

SUPPORT SYSTEMS COMMON TO MOST REFINERIES 535

Figure 13.6. Floating roof tank.

in the roof also provides access for sampling the tank contents. In oil reﬁning this

type of tank is used for the storage of gas oils, diesel, light heating oil, and the very

light lube oils (e.g., spindle oil).

In keeping with the company’s ﬁre protection policy tanks containing ﬂammable

material will be equipped with foam and ﬁre water jets located around the base of

the roof. All storage tanks containing ﬂammable material and material that could

cause environmental damage are contained within a dyked area or bund. The size of

the bunded area is ﬁxed by law and is usually such as to contain the total contents of

one of the tanks included in the area. The number of tanks per bunded area is also

ﬁxed by legislation.

Floating roofed tanks

Light volatile liquids may also be stored at essentially atmospheric pressure by the

use of “Floating Roof ”tanks. A diagram of this type of storage tank is also given in

Figure 13.6.

The roof of this tank literally ﬂoats on the surface of the liquid contents of the tank. In

this way the air space above the liquid is reduced to almost zero, thereby minimizing

the amount of liquid vaporization that can occur. The roof is specially designed

for this service and contains a top skin and a bottom skin of steel plate, held together

by steel struts. These struts also provides strength and rigidity to the roof structure.

The roof moves up and down the inside of the tank wall as the liquid level rises when

ﬁlling and falls when emptying. The roof movement is enhanced by guide rollers

between the roof edge and the tank wall. A scraper ring around the edge of the roof

536 CHAPTER 13

top and pressed tightly against the tank wall ensures a seal between the contents and

the atmosphere. It also provides additional guide to the roof movement and stability

to the roof itself.

When the tank reaches the minimum practical level for the liquid contents the roof

structure comes to rest on a group of pillars at the bottom of the tank. These provide

the roof support when the tank is empty and a space between the roof and the tank

bottom. This space is required to house the liquid inlet and outlet nozzles for ﬁlling

and emptying the tank which, of course, must always be below the roof. The space is

also adequate to enable periodic tank cleaning and maintenance.

A hinged drain line running inside the tank from the roof to a “below grade”sealed

drain provides the facility for draining the roof of rain water. The hinges in the drain

line allows the line to move up and down with the roof movement. A pontoon type

access pier from the platform around the perimeter at the top of the tank provides

access to the sample hatch located at the center of the roof. This “pontoon”also

moves upward and downward with the roof movement. Automatic bleeder vents are

provided on all ﬂoating roof tanks. They vent air from under a ﬂoating roof when the

tank is being ﬁlled initially from empty. After the liquid rises high enough to ﬂoat

the roof off its supports, the vent automatically closes. Likewise when the tank is

being emptied, the vent automatically opens just before the roof lands on its support,

thereby preventing the development of a vacuum under the roof. Other accessories

include rim vents, ﬂoat gauges, anti rotation devices, and manholes.

Liquids stored in this type of tank have relatively high volatilities and vapor pressures

such as gasoline, kerosene, jet fuel, and the like. In oil reﬁning the break between

the use of cone roof tank and ﬂoating roof is based on “ﬂash point”of the material.

Flash point is that temperature above which the material will ignite or “ﬂash”in the

presence of air. Normally this break point is 120

◦

Pressure storage

Pressure storage tanks are used to prevent or at least minimize the loss of the tank

contents due to vaporization. These types of storage tanks can range in operating

pressures from a few inches of water gauge to 250 psig. There are three major types

of pressure storage. These are:

Low-pressure tanks—These are dome roofed tanks and operate at a pressures of

between 3 ins water gauge and 2.5 psig.

Medium pressure tanks—These are hemispheroids which operate at pressures be-

tween 2.5 and 5.0 psig, and spheriodal tanks which operate at pressures up to 15 psig.

High-pressure tanks—These are either horizontal “bullets”with elipsoidal or

hemispherical heads or spherical tanks (spheres). The working pressures for these

SUPPORT SYSTEMS COMMON TO MOST REFINERIES 537

types of tanks range from 30 to 250 psig. The maximum allowable is limited by

tank size and code requirements. For a 1,000 bbl sphere, the maximum pressure is

215 psig, for a 30,000 bbl it is 50 psig. These pressure limits can be increased if

the tank is stress relieved.

Although it is possible to store material in tanks with pressure in excess of 250

psig normally when such storage is required refrigerated storage is usually a better

alternate.

Heated storage tanks

Heated storage tanks are more common in the petroleum industry than most others.

They are used to store material whose ﬂowing properties are such as to restrict ﬂow at

normal ambient temperatures. In the petroleum industry products heavier than diesel

oil, such as heavy gas oils, lube oil, and fuel oil are stored in heated tanks.

Generally speaking tanks are heated by immersed heating coils or bayonet type im-

mersed heaters. Steam is normally used as the heating medium because of its avail-

ability in petroleum complexes. Very often where immersed heating is used the tank

is agitated usually by side located propeller agitators for large tanks. Where external

circulating heating is used for tanks, the contents are mixed by means of jet mixing.

Here the hot return stream is introduced into the tank via a specially designed jet noz-

zle as shown in Figure 13.7. External tank heating is used when there is a possibility

of a hazardous situation occurring if an immersed heater leaks.

Figure 13.7. A jet mixing nozzle.

538 CHAPTER 13

Calculating heat loss and heater size for a tank

Heat loss and the heater surface area to compensate for the heat loss may be calculated

using the following procedure:

Step 1. Establish the bulk temperature for the tank contents. Fix the ambient air

temperature and the wind velocity normal for the area in which the tank is to be sited.

Step 2. Calculate the inside ﬁlm resistance to heat transfer between the tank contents

and the tank wall. The following simpliﬁed equation may be used for this:

= 8.5(t/µ)

0.25

where

=Inside ﬁlm resistance to wall in Btu/hr sqft

◦

t = Temperature difference between the tank contents and the wall in

◦

µ =The viscosity of the tank contents at the bulk temperature in cps.

The heat loss calculation is iterative with assumed temperatures being made for

the tank wall.

Step 3. Using the assumed wall temperature made in step 2 calculate the heat loss to

atmosphere by radiation using Figure 13.8. Then calculate the heat loss from the

tank wall to the atmosphere using Figure 13.9. Note the temperature difference in

this case is that between the assumed wall temperature and the ambient air tem-

perature. Correct these ﬁgures by multiplying the radiation loss by the emissivity

factor given in Figure 13.8. Then correct the heat loss by convection ﬁgure by the

factors as described in item 4 below.

Step 4. The value of h

read from Figure 13.9 is corrected for wind velocity and for

shape (vertical or horizontal) by multiplying by the following shape factors:

Vertical plates 1.3

Horizontal plates 2.0 (facing up)

1.2 (facing down)

Correction for wind velocity use

= F

+ F

where

=wind correction factor

=wind factor @ 200

◦

F calculated from:

=(MPH/1.47)

0.61

=Read from Figure 13.10

Then the corrected h

is:

× shape correction ×Fw.

SUPPORT SYSTEMS COMMON TO MOST REFINERIES 539

Figure 13.8. Heat loss by radiation.

Step 5. The resistance of heat transferred from the bulk of the contents to the wall

must equal the heat transferred from the wall to the atmosphere. Thus:

Heat transferred from the bulk to the wall = ‘a’

= h

from step 2 × t in Btu/hr · sqft.

where t in this case is (bulk temp—assumed wall temp)

540 CHAPTER 13

Figure 13.9. Heat loss to atmosphere by natural convection.

Figure 13.10. Plot of ‘F

’v ersus surface temperature.

SUPPORT SYSTEMS COMMON TO MOST REFINERIES 541

Heat transferred from the wall to the atmosphere = ‘b’

= (h

+ hr) × t in Btu/hr sqft

where t in this case is (assumed wall temp—air temp).

Step 6. Plot the difference between the two transfer rates against the assumed wall

temperature. This difference (‘a’ − ‘b’) will be negative or positive but the wall

temperature that is correct will be the one in which the difference plotted = 0.

Make a last check calculation using this value for the wall temperature.

Step 7. The total heat loss from the wall of the tank is the value of ‘a’or ‘b’calculated

in step 6 times the surface area of the tank wall. Thus:

wall

= h

× t × (π D

tank

× tank height) in Btu/hr.

Step 8. Calculate the heat loss from the roof in the same manner as that for the wall

described in steps 2–7. Note the correction for shape factor in this case will be for

horizontal plates facing upward, and the surface area will be that for the roof.

Step 9. Calculate the heat loss through the ﬂoor of the tank by assuming the ground

temperature as 50

◦

F and using;

= 1.5 Btu/hr sqft

◦

Step 10. Total heat loss then is:

Total heat loss from tank = Q

wall

+ Q

roof

+ Q

ﬂoor

Step 11. Establish the heating medium to be used. Usually this is medium pressure

steam. Calculate the resistance to heat transfer of the heating medium to the outside

of the heating coil or tubes. If steam is used then take the condensing steam value

for h as 0.001 Btu/hr sqft

◦

F. Take value of steam fouling as .0005 and tube metal

resistance as 0.0005 also. The outside fouling factor is selected from the following:

Light hydrocarbon = 0.0013

Medium hydrocarbon = 0.002

Heavy Hc such as fuel oils = 0.005

The resistance of the steam to the tube outside =

h + R

where R = r

steam fouling

tube metal

outside fouling

Step 12. Assume a coil outside temperature. Then using the same type of iterative

calculation as for heat loss, calculate for ‘a’as the heat from the steam to the coil

outside surface in Btu/hr sqft. That is

‘a’ = h × 

Calculate for ‘b’as the heat from the coil outside surface to the bulk of the tank

contents. Use Figure 13.11 to obtain ho and again ‘b’is h

× 

where the 

the temperature between the tube outside and that of the bulk tank contents. Make

further assumptions for coil outside temperature until ‘a’ = ‘b ’.

Step 13. Use ‘a’or ‘ b’from step 12 which is the rate of heat transferred from the

heating medium in btu/hr sqft and divide this into the total heat loss calculated

542 CHAPTER 13

Figure 13.11. Convection heat transfer coefﬁcient.

in step 10. The answer is the surface area of the immersed heater required for

maintaining tank content’s bulk temperature.

An example calculation using this technique is given as Appendix 13.1 at the end of

this chapter.

Product blending facilities

Blending is the combining of two or more components to produce a desired end

product. The term in reﬁnery practice usually refers to process streams being combined

to make a saleable product leaving the reﬁnery. Generally these include gasolines,

middle distillates such as: jet fuel, kerosene, diesel, and heating oil. Other blended