Daqing Petroleum Institute Undergraduate Paper

Daqing Petroleum Institute Undergraduate Paper

Chapter 1 Classification of Tank

1.1What is chemical engineering?

In a wider sense engineering can be defined as a scientific presentation of the techniques and facilities used in a particular industry. For example mechanical engineering refers to the techniques and facilities employed to introduce machines.

Chemical engineering encompasses the chemical process of raw materials, based on chemical and physicochemical phenomena of high complexity. Chemical engineering is a branch of engineering concerned with the converting raw materials, such as crude oil. Commonly these materials are liquids, powders, or gas. We need process industry to provide us food (and drink), clothing, shelter, building materials, etc. As this industry forged ahead, new date, new relationships and new generalizations were more and more added chemical engineering, many branches in their own right have separated from the main stream of chemical engineering, such as process and plant design, automation, chemical process simulation and modeling, etc.

1.2 What is process equipment?

Chemical processes may consist of widely varying sequences of steps and the principles of which are independent of the material being operated upon and of other characteristics of the particular system. Any process can be resolved into unit actions, pulverizing, roasting, mixing, heating, roasting, absorbing, condensing, lixiviating, precipitating, crystallizing, filtering, dissolving, electrolyzing and so on. Vessels and machineries are used to finish these “single “ tasks. It can be classified into process vessels and process machinery. And process vessels are often static and process machinery is often moveable equipment.

1.3 Classification of process vessel

There are many methods to classify vessels. According to the pressure they bear, process vessels can be classified as Low-pressure vessel (L for short), Middle-pressure vessel (M), High-pressure vessel (H), and Unique-pressure vessel (U).

According to their usage, as Reaction vessels (R), heat -Exchangers (E), Separation vessels (S), Containers.

Due to their installation method, as Fixed pressure vessels and Removable pressure vessels.

1.4 Classification of tank

Tanks are containers. Tank can be divided into the following types as show in table 1-1

Table 1-1 Classification of tank

TYPE / TYPICAL CHARACTERISTICS OR SPECIFIC TYPE
Square tank / Square section, with flat bottom or conical bottom.
Cylindrical horizontal tank / Circle section, with flat head, dish head, ellipsoidal head or spherical head.
Cylindrical vertical tank / Low-pressure gas cupboard / The wet type gas cupboard[①].
The dry type gas cupboard (piston type and dissepimental type).
Fixed-head tank / Conical vault tank (self-supported, column-supporting).
Vault tank (single-layer wall, or double-layer wall).
The umbrella shape top tank.
Floating-head tank / With single-tray or two trays.
Lifting head tank / The head is sealed with liquid, and can be lifted to increase the volume.
Spherical tank / The wall is a single layer or layers with the same thickness.
Special tank / Semi-ellipsoidal, multi-cylindrical and multi-spherical surfaces
High frame sink / Sink is built on far off the ground, and water could easily flow.

Among them the most common and extensively used group is the vertical cylindrical tank. Its bearing load is low, and basically it belongs to the ordinary pressure vessel. And the vault tank is the tank that can bear the biggest pressure whose design pressure is generally 8Kpa and –0.5Kpa. The merit of this kind of this kind tank is: it is easy to fabricate with low cost. And this construction is often used in small-size tank whose volume is below 10000m3. In this paper, the tank designed is vertical cylindrical tank whose design pressure is 0.1Mpa,and the design includes tank wall material selection, tank wall thickness calculation, vault design, bottom plate design, low temperature heat insulation and so on.

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Daqing Petroleum Institute Undergraduate Paper

Chapter 2 Tank Structure

With cylindrical shell, formed heads, blind flanges, cover plates, flanges, openings and nozzles, supports, foundation, floor are the basically the same. Take the vault tank as example. Tank structure design includes materialselection, tank foundation design, tank floor design,tank wall design and tank head design. Typical components of tank s are described below.

2.1 Shell

Cylindrical shell is frequently used for constructing pressure vessels in the petrochemical Industry. It is easy to fabricate and install and economical to maintain. The required thickness is controlledmainly by internal pressure, applied loads, external pressure, thermalstress, discontinuity stresses.

2.2 Head

A large variety of end closures and transition sections are available to the design engineer. The selection depends mainly on method of manufacture, material cost and space restriction.

2.3 Blind flanges, cover plates and flanges

One of the more common types of closures for pressure vessels is the unstayed flat head or cover. This may be either integrally formed with the shell or welded to the shell or it may be attached with bolts or some quick-opening device. It may be circular, square, rectangular, or some other shape.

2.4 Opening and nozzles

All process vessels requireopenings to get the contents in and out. For some tanks, access is made through large openings in which the entire head of a section of the shell is removed. However, for most tanks, the contents enter and exit throughopenings in the heads and shell to which nozzles and piping are attached. Other openings such as man-way, hand-hole, and openings required for cleaning and draining or inspecting the tanks from outside.

2.5 Support

Many vertical tanks are supported with skirts. They generally transfer the loads from the vessel by shear action, to the foundation through anchor bolts and bearing plates. Leg-supports are usually used for lightweight vertical vessels. Support skirts are often used, too. Horizontal vessels are normally supported with saddles, added by stiffening rings.

2.6 Foundation

It provides a plate to bear the weight and all external loads. The foundation can be divided to ferroconcrete, or many layers as white soil layer, dust gravel layer, sand cushion layer and pitch sand layer from top to bottom.

2.7 Material selection

We have to consider both economic and strength conditions. We employ many effective technologies to build strong and economic vessels. But strength conditions are of first importance.

We’ll discuss this topic more specifically in part one chapter 3.

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Daqing Petroleum Institute Undergraduate Paper

Chapter 3 Material Selection

3.1 General Considerations

Vessel engineers must be familiar with commonly used construction materials to be able to specify them correctly on engineering drawings or in material specifications for a particular job. Generally, process equipment is designed for a certain minimum service life under specific operation conditions.

The selection of construction materials for Code pressure vessels has to be made from Code approved materialspecifications.

There are many factors supported by experience and laboratory test results that must be considered in selecting the most suitable materials. They include the following:

1). Strength requirements for design temperature and pressure;

2). Corrosion resistance in the service corrosive environment;

3). Cost;

4). Ready market availability;

5). Fabricability;

6). Quality of future maintenance.

We could look up Industry dictionary to decide the strength of a specificmaterial.At the same time we can use this data to make sure the design acceptable.

3.2 Properties of Materials

The final strength of any material used component depends on its mechanical and physical properties after it has been subjected to one or more different manufacturing processes. Also there are several properties that determine the suitability of the material in its initial state for any particular manufacturing process.

We just focus on the properties that may affect the tank usage. Generally speaking, they are:

(1) Strength- capacity to resist fail of a component or an element.

(2) Rigidity- capacity to resist deformation of a component or an element.

(3) Stability- capacity to resist original state in the equilibrium of a component or an element.

Specifically, these properties include: proportional limit, elastic limit, yielding strength, ultimate strength, residual relative elongation, permanent relative reduction of area. The relationship between these properties can be show in the following table:

Table 3-1 Basic Material Properties

Strength / Rigidity / Stability
Proportional limit, elastic limit, yield strength, ultimate strength / Residual relative elongation, permanent relative reduction of area / Stability has more to do with the shape than the material itself.

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Daqing Petroleum Institute Undergraduate Paper

Chapter 4 Tank Manufacture

4.1 Design

A good project starts with the designer. Designing is very important in defining a project. In a wider sense it determines:

(1) Selection of a Plant Site

When selecting the plant location we consider: raw material, market, energy sources, climate, transportation, water supply, impact on environment, labor source, land saving, safety and so on.

(2) Distribution of the Building

General layout of plant should be satisfied with three aspect requirements: safety, development and convenience of manufacturing.

(3) Safety and Environment protection

How can we reduce the amount of waste that is produced? To protect environment we have to consider the following points: Life cycle analysis, Manufacturing with minimal environment impact, Environmentally friendly products, Recycling.

We must take safety and explosion into account when designing. It could not cause serious pollution on air, water, and human health first of all.

(4) Public Engineering

It includes: power, water supply and drainage, heating and ventilation.

(5) Automatic Controlling

We adopt automation to save labor and be safer and economical. It mainly includes the following aspects: automatic check system design, automatic signal interlocking protection system design, automatic controlling system design, automatic regulating system design, etc.

(6) Architecture

It includes convenience of transport, convenience of source and structural strength.

4.2 Construction

When constructing the tank, we must fullymeet all technical requirements. Therefore, we must keep an eye on all procedures of manufacturing, construction, installation, welding, supervising. We should take care even at the beginning of preparing.

Take care of working shop. For better product quality and controlling, we’d better prepare all the material in the workshop and install them in field right away.

4.3 Welding

4.3.1 Definition of Welding

Nowadays welding is the most commonly used method of fabrication of pressure vessel shells. Structural, non-pressure parts (such as stiffening rings, lifting rings, support clips for piping, internal trays, and other equipment) are usually attached to vessel wall by welding. Welded joints, instead of bolted joints, are sometimes used for piping-to-vessel connections to obtain optimum leak-proof design where a desirable, as with lethal fluids or in high temperature service.

A structure whose component parts are jointed by welding is called a weldment. It isobtainedin localizedunion of metal achieved in plastic and molten states.

4.3.2 Classification of welding

There are three welding methods: Forge welding, Fusion welding, Pressure welding. And the most widely used industrial method is ARC welding. Heat is produced by electric arc, metal is fused at this high temperature of 1500~1800℃.

Welding can only adopted while the Carbon manganese content is less than 0.35%. This is because carbon is an effective steel hardener. Cooling after welding causes dimensional changes in the weld due to temperature reduction and phase change. These changes may result in occasional crack in the weld or the heat-affected zones (HAZ).

For design purpose, we often divide welds into three basic types, calling for different design methods and design stresses: groove, fillet and plug welds. Butt or tee joints with V or double-bevel groove welds are used extensively, particularly where cyclic loads are expected. They are easier to design than fillet welds, but harder to fabricate. The edge preparation required depends on the plate thickness and is usually specified by the applicable code.

4.3.3 Defects of welding

Welding unavoidably causes defects. And there are seven typical sorts of defects:

1)Poor weld shape caused by misalignment;

2)Cracks in welds or heat-affected zone;

3)Pinholes on the weld surface;

4)Slack inclusions or porosity in the form of voids;

5)Incomplete fusion between weld heads or weld and base metal;

6)Lack of insufficient penetration of the metal in joints;

7)Undercutting, an intermittent or continuous groove adjacent to the weld left unfilled by weld metal;

Each step of fabrication may cause some sort of defects. However, if the defect is acceptable, we do not take it as “defect”.

4.3.4 Examination for Welding

Weld examination becomes very important to ensure good quality. And nondestructive testing is widely used. Visual testing (VT), Penetrant Testing (PT), Magnetic particle testing (MT), Radiographic testing (RT), Ultrasonic testing (UT), Eddy current testing (ET), Thermal infrared testing (TIR), Acoustic emission testing (AE) are the most often used nondestructive testing (According to reference 9).

Additionally, this high-temperature welding process unavoidably cases harmful and complex residual stresses. And it becomes dangerous when the weld residual stress is superposed on the stress caused by an external load and exceeds the yield point of the material. This is most true for ductilematerials. Often we adopt preheat treatment and post-weld heat-treating (PWHT) to minimize the residual stress.

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Daqing Petroleum Institute Undergraduate Paper

Chapter 5 Tank Foundation

The foundation of oil tank directly stands up to the oil weight and the tank weight, and delivers the weight to the soil, at the same time it plays important part in pledging oil tank installation precision, completely cutting off groundwater, keeping the tank floor dry and preventing the steel floor plate from being corroded.

The following are the most outstanding characteristics of the foundation.

1. It doesn’t require much strength of the foundation. A tank with the volume of 100~20000m3 and the height of 6~19m just requires the capability of bearing 80~200KPa;
2. Sedimentation is not very strict. Even if a tank has fallen down 1.5~2m, it can still be in use. But the tank must fall down uniformly. Otherwise the sedimentation will cause stresses and destroy the tank.
3. The weight of the tank is “a little extra” compared with the weight of the liquid when it’s full. Most often we do not consider the tank steel weight.
4. The tank is flexible itself and able to absorb a certain amount of unbalanced sedimentation. This is strongly required in the coastland where the soil is often soft.

The foundation is situated on the base of oil tank. In order to stabilize the foundation, the bearing limit of the base soil must not be lower than the allowance value, and the geology circumstances must be uniform to prevent the oil tank from sinking and destructing. The allowance value of the base soil bearing limit is determined by the height of oil tank, generally it is 10 to 18 t/㎡. For the district that can’t satisfy the above conditions must be handled specially.

The foundation building methods of oil tank are different because of different soil in building district. Generally, in inland the foundation can be divided to white soil layer, dust gravel layer, sand cushion layer and pitch sand layer from top to bottom. This may take the cheapest cost.

The white soil layer is gained by pressing the original soil solidly after removing the surface humus.

The dust gravel layer is achieved by mixing lime, soil with sand or crushed stone, small and rough sand according to a fixed proportion and pressing them solidly. The effect of this layer is to improve the resistance ability of the foundation and increase the stability of the foundation.

Sand cushion layer is made of rough sand and middle-sized sand in which the mud is not more than 5%, and it appears as a conical contour with high center and low periphery, the thickness is 20 to 30cm. The capability of sand is slight and the height of groundwater in the sand layer is not more than 15cm, keeping tank floor dry. Moreover, the cohesion between the grains of sand is local, and the sand possesses elasticity, so it can evenly distribute the loads from the tank floor to the base soil. The function of pitch layer is to prevent water from going up and protect the floor plate from being corroded, the thickness of it is generally 8-10cm, and it is spread into the conical contour according to shape of the sand cushion layer.

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Daqing Petroleum Institute Undergraduate Paper

Chapter 12 Tank Wall Thickness Calculation

Basically we decide the thickness by calculating the stress condition and check the thickness by stability or experience data. And there is something more to consider along with thickness more than calculation.

Reference (1), page 205, table 11-5,gives the minimized thickness according to the material and diameter. So we see the minimum thickness is 6 mm, as the diameter is21.5m, between 15 and 36m.