Hazardous area classification, Signal isolation, methods of protection in the dangerous atmosphere.

Classification of hazardous areas

The codes used by the different manufacturers subdivide hazardous areas into zones.

Zone 0: In this zone, an explosive gas-air mixture is present continuously or for long periods of time.

Zone 1: In this zone, an explosive gas-air mixture is likely to be in normal operation.

Zone 2: In this zone, an explosive gas/air mixture is unlikely to be in normal operation, and if it does, it will only be for a short period of time.

Signal isolation:

When working in the field of chemical or petrochemical industries, the explosion risk factor must be taken into account.

As a consequence, it is necessary to install a system that exits in the safe zone from the dangerous zone. That means the equipment installed in the panel and in the field respectively. There are basically two types of signal isolation.

Flameproof (In America explosion-proof) enclosure.

Intrinsically safe enclosure.

Flameproof:

The first of them is used whenever the conditions of the current, intensity of heat produced by the connected elements, exceed the limits allowed by the CENELEC Standards, or any other that is used in each specific case.

This type of installation requires that the connections be made through a box.

Intrinsic Safety:

It is the most widely used, is based on installing primary and final elements with intrinsic safety certification, that is, that their parameters do not exceed the limits set by the CENELEC Standards (in English: European Committee for Electro technical Standardization), such as:

Voltage: 30 volts

Current:  50 milliamps

Depending on the technology used to develop the engineering of the system, the mentioned elements can be connected through Zener safety barriers or galvanic isolators.

The digital signals are connected through suitably certified serial repeater relays. The following sections describe, in a simplified way, the characteristics of each of the connection types mentioned. Simply put, an intrinsically safe system can be divided into three parts.

Interface, in a safe area, between the non-intrinsically safe equipment located on the panel and the intrinsically safe system located on the ground, an example is the Zener barrier.

Wiring, boxes, and accessories to connect the interface with equipment located in a hazardous area plant located equipment. For example, a 2-wire transmitter with 4 to 20 mA of serial.

Protection methods in dangerous atmospheres.

The CENELEC EN-50014 standards are bound by the features that are common to all protection methods.

In America, the term explosion-proof is used for the same concept as in Europe it is called flameproof. So care must be taken to explain it correctly

The most common protection methods are described below, with the indication of the corresponding symbols.

Pressurization (Ex p): It is a type of protection by which the atmosphere surrounding the enclosure of an appliance is prevented from penetrating inside it, maintaining an inert protection gas inside at a pressure higher than the one that surrounds the box. It is used in analytical equipment and other areas where the use of other techniques would be impossible.

Explosion-proof enclosures (Ex d): It is the flameproof protection technique, in which the parts that can ignite an explosive atmosphere are placed inside an enclosing box capable of withstanding the pressure developed during the internal explosion of the explosive mixture, and avoiding the transition of the explosion to the dangerous atmosphere surrounding the box.

This technique is used for instruments whose power level is very also and cannot be eliminated.

Increased safety (Ex e): This technique obtains safety by applying additional measures to the equipment to increase the protection against the possibility of excessive temperature and the occurrence of arcs or sparks.

In practice, it translates into rugged construction, additional insulation, and new mechanical protection.

Intrinsic safety (Ex i): It is a protection technique whereby devices that contain circuits designed under this concept are incapable of occasionally exploding the surrounding atmosphere, limiting energy and surface temperature. In turn, it has two applicable standards, such as are.

Where security is maintained with up to two failures produced.

Where security is maintained with a single failure.

Basics of Electrical ground system.

Earth ground installation. A good GROUND system is very important in any electrical installation to protect any of our electrical equipment, electronics, machinery, etc. against downloads electrical.

If an office or company already has a physical ground, then one knows the benefits that this has provided you as savings on repairs in our equipment and savings by not stopping their operations due to damaged equipment.

What is Earth, Ground, and Neutral?

Earth ground and neutral are different things. The neutral is the reference of the phases of our three-phase electrical system, is the “Negative” of the cable that carries the current.

The physical earth is the means of the discharge of the current that could be in the chassis of the equipment or in the case of occurring a short circuit is discharged by that means. To install the earth ground there are several methods.

How to install earth ground?

Electrical ground installation

The most common and most used method is to bury a rod in the ground. It is a conventional physical land, it consists of a rod covered by a thin layer of copper that is buried at a certain depth and covered by salts minerals, it will suffer deterioration over time due to the natural corrosion of the soil, thus generating a loss of the protection capacity, for which it is necessary to renew it.

If you want to have a better ground system you can place two or three rods at different points and join them between yes with a No. 6 electrical conductor.

A better method of installing a physical ground is to place a mesh spreading over a ground surface in mesh forms a bare electrical conductor of good thickness and cover it in the ground to a small depth.

The contact surface and the arrangement of the mesh generate greater protection than a conventional rod. For both cases, it is very important that the physical ground is one to neutral on the main switch. This union will allow having protection against electric shock and will help have a firmer and more stable three-phase electrical system.

Benefits of the ground system

•             Always remember that a good ground system is very much important in a safe electrical installation.

•             It is recommended not to forget that these copper pipes and other elements require maintenance as they suffer wear over time.

•             The reason for installing earth ground protection is as important as the purchase of the voltage regulation.

•             To protect your equipment against voltage fluctuations.

Actuator types, Hydraulic and Pneumatic.

Actuators are devices capable of generating a force from liquids, gases, and electrical energy. The actuator receives a command signal from the controller and gives an output necessary to activate a final control element such as valves.

They can be

  • Hydraulic
  • Pneumatic

Hydraulic actuators 

They are used when high power is needed, hydraulics require too much equipment for power supply, as well as periodic maintenance. On the other hand, the applications of pneumatic models are also limited from the point of view of precision and maintenance.

The work done by a pneumatic actuator can be linear movement or rotary.

The linear movement is obtained by piston cylinders (these also provide the rotary movement with a variety of angles by means of rack and pinion type actuators).

We also find pneumatic actuators of continuous rotation (pneumatic motors), combined movements, and even some mechanical transformation of movement that makes it seem of a special type.

Pneumatic actuators

Although the function of the pneumatic and hydraulic actuators are identical, they are classified into

Linear actuators.

Rotary actuators.

Linear Pneumatic Actuators

The pneumatic cylinder consists of a closed cylinder with a piston inside that slides and transmits its movement to the outside through a rod. It is made up of the rear and front covers.

Pneumatic cylinders, regardless of their constructive form, represent the most common actuators used in pneumatic circuits. There are two fundamental types from which special constructions derive.

Single-acting cylinders, with an air inlet to produce a working stroke in one direction.

       Double-acting cylinders, with two air inlets to produce exit and backward work strokes.

Singleacting cylinders

A single-acting cylinder performs work only in one direction. The plunger is made to return by means of an internal spring or by some other external means such as loads, mechanical movements, etc. Its work can be of the “normally in” or “normally out” type.

Single-acting cylinders are used for

clamping,

marking,

ejecting, etc.

They consume less air than a double-acting cylinder of the same size. However, there is a reduction in momentum due to the counterforce of the spring, so a somewhat larger internal diameter may be necessary to achieve the same force.

Also, the adaptation of the spring results in a longer overall length and a limited stroke length, due to dead space.

Types of single-acting cylinders:

Piston cylinders,

diaphragm cylinders,

roll-up diaphragm cylinders.

Double-acting cylinders 

Double-acting cylinders are those that perform both their forward and backward strokes by the action of compressed air. Its name is due to the fact that they use both faces of the plunger (air in both chambers), so these components can do work in both directions.

Its internal components are practically the same as those of simple effect, with small variations in its construction.

Plunger operated Pneumatic Cylinder

The field of application of double-acting cylinders is much broader than that of single-acting cylinders, even when no effort is needed in both directions. This is because, as a general rule (depending on the type of valve used for control), double-acting cylinders always contain air in one of their two chambers, so positioning is ensured.

In order to carry out a certain movement (forward or backward) in a double-acting actuator, a pressure difference must exist between the chambers. In short, we can say that double-acting linear actuators are the most common components in pneumatic control. This is due to:

  •  It is possible to carry out work in both directions (forward and backward strokes).

  •  No force is lost in the actuation due to the absence of spring in opposition.

  •  For the same cylinder length, the double-acting stroke is greater than in the single-acting arrangement, as there is no housing volume.

Other types of cylinders:

Pneumatic bellows cylinder.

Also known as a bellows air motor, it incorporates a double-acting cylinder, a directional control valve actuation system, and two forward and reverse speed regulation screws.

·Pneumatic impact cylinder

The rod of this cylinder moves at a high speed of the order of 10 m / s and this energy is used to carry out marking work on engine benches, wooden profiles, electromechanical components, and work on time-stamping dams, stamping, riveted, bent, etc.

Rod less Pneumatic Cylinder

When the available cylinder space is limited, the pneumatic rod less cylinder is the choice. It can have a relatively long stroke of about 800mm and greater.

Guided pneumatic cylinder

One of the problems that conventional cylinders present is the rotational movement that the rod can suffer, since the piston, the rod, and the cylinder liner are circular in section, so none of them prevent rotation. In some applications, free rotation is not tolerable, so an anti-rotation system is necessary.

One of the systems that, apart from the anti-rotation function, has other advantages is the guided pneumatic cylinder that contains two or more pistons with their rods, which gives rise to a force twice that of conventional cylinders.

They consist of two or more double-acting cylinders coupled in series. Two cylinders with different strokes make it possible to obtain four different piston rod positions.

Tandem cylinders

It is made up of two double-acting cylinders that form a unit. By simultaneously applying pressure to the two pistons, a force of almost twice that of a normal cylinder for the same diameter is obtained on the rod.

Rotary pneumatic actuators

Rotary or rotary actuators are responsible for transforming pneumatic energy into rotational mechanical energy. Depending on whether the turning mobile has a limited angle or not, the two large groups to be analyzed are formed:

Limited turn actuator

They are those that provide turning movement but do not produce a revolution (except for some particular mechanics such as rack and pinion). There are single and double effect arrangements for turning angles of 90º, 180º …, up to a maximum value of about 300º (approximately).

Pneumatic motors

They provide a constant rotary motion. They are characterized by providing a high number of revolutions per minute.

LIMITED ROTATION ACTUATORS

Vane actuator

The vane-type rotary actuator is perhaps the most representative of the group of limited-turn actuators. These actuators perform a turning movement that rarely exceeds 270º, incorporating mechanical stops that allow the regulation of this turn.

They are made up of a casing, inside which is a palette that delimits the two chambers. Integrated with this pallet is the shaft, which runs through the outer casing. It is precisely on this axis that we get the work, in this case in the form of limited angular motion.

As we can see in the below figure, the operation is similar to that of double-acting linear actuators. When applying compressed air to one of its chambers, the blade tends to rotate on the axis, as long as there is a pressure difference with respect to the opposite chamber (generally communicated with the atmosphere). If the position is reversed, a turning movement in the opposite direction is achieved.

These components have advantages inherent to the latest generation components, such as damping at the end of travel, the possibility of magnetic position detection (mechanical or magnetic), etc. The mechanical detection is carried out by means of external mobile elements adjustable in degree by means of the graduated vernier.

The cylinders that function as rotary actuators, of limited turn, are the rotary piston-rack-pinion cylinder in which the linear movement of the piston is transformed into a rotary movement by means of a rack and pinion assembly and the Rotary vane cylinder of double-acting for angles between 0 ° and 270 °. In the following figure the piston-rack-pinion cylinder:

LINEAR HYDRAULIC ACTUATORS

Linear motion hydraulic cylinders are commonly used in applications where the thrust force of the piston and its displacement are high.

Hydraulic cylinders can be

Single-acting,

Double-acting

Telescopic.

Single-acting hydraulic cylinder

In the first type, the hydraulic fluid pushes the cylinder piston in one direction and an external force (spring or gravity) retracts it in the opposite direction. The cylinder body is the tubular outer case and contains the piston, piston seal, and rod. “Caliber” is the term used to indicate the diameter of the piston. The piston end of the cylinder (sometimes called the “blind end”) is known as the head end. The end from which the stem extends and retracts is known as the stem end.

The double-acting cylinder

It uses the force generated by the hydraulic fluid to move the piston in both directions, using a solenoid valve. The double-acting cylinder is the most common hydraulic actuator used today and is used in implement, steering, and other systems where the cylinder is required to operate in both directions.

A cylinder bore is a term that indicates the internal diameter of the cylinder. A large-bore cylinder produces a greater volume per unit length than a small bore cylinder. To move a piston the same distance, a large-bore cylinder needs more oil than a smaller bore cylinder. Therefore, for a given flow regime, a large-bore cylinder moves more slowly than a small bore cylinder.

The effective area of a cylinder is the area of the piston and piston seal on which the oil acts. Because one end of the rod is attached to the piston and the opposite end extends out of the cylinder, the effective area of the end of the rod is less than the effective area of the head end. The oil does not act against the area of the piston covered by the rod joint.

Its design allows the oil pressure to extend the seal against the cylinder wall, so that the higher the pressure, the greater the sealing force. The head end seal (ring seal) prevents oil from escaping between the stem neck and the cylinder wall.

The seals are made of polyurethane, nitrile, or Viton. The material must be compatible with the fluids used and the operating conditions.

Shock absorbers

The figure shows a cylinder with shock absorbers.

When a moving cylinder comes to a dead-end (such as at the end of the cylinder’s stroke), the action it experiences is known as “shock loading.” When a cylinder is subjected to a shock load, shock absorbers are used to minimize the effect.

As the piston approaches the end of the stroke, the damper moves within the return oil passage and restricts the flow of return oil from the cylinder. The restriction generates an increase in the return oil pressure between the return oil passage and the piston. The increase in oil pressure produces a “damping effect” that reduces the movement of the piston and minimizes the shock that occurs at the end of the stroke.

Some cylinders may require a head-end damper, while others may require both head end and rod end dampers.

What is SIS, SIF, and SIL ?

Importance of Safety

Safety is increasingly important for the high-risk industry like oil, gas, chemicals and petrochemicals to adequately implement and follow safety programs. This can be achieved by adhering to regulations, which ensure safe processes and higher productivity.

Risk Involvement

The high-risk industry is one, where processes handle unstable reactions that can get out of control quickly and frequently.

A series of unforeseen conditions or events can trigger incidents with serious consequences for personnel, facilities and production.

Due to the above, the need is created to implement equipment and systems (layers of protection) that minimize the probability of an unwanted event or that minimize the risk.

Regarding these layers of protection, we can find those that involve both hardware and software such as:

  • Alarm Systems
  • Instrumented Security Systems

SIS

(Safety Instrumented System)

Instrumented system used to perform one or more Safety Instrumented Functions (IEC 61511-1: 2016 – 3.2.67).

Safety Instrumented System (SIS)

The SIS, in general, is made up of several Security Instrumented Functions (SIF) and all the common services necessary for its operation. It provides the basic common support services to host them, that is, the

  • Physical housing (for example, chassis for I/O cards or common processor housing).
  • Logical housing (for example, application program housing), common service such as power supply, etc.

SIF

(Safety Instrumented Function)

Safety function to be implemented by a Safety Instrumented System (SIS) (IEC 61511-1: 2016 – 3.2.66).

The SIF is considered a system formed by at least three (3) sub-systems:

Safety Instrumented Function (SIF)
  • The detection sub-system (One or more measurement or detection elements).
  • The logical sub-system (commonly a Programmable Safety Logic Controller).
  • The final element (which can be made up of one or more valves, motors, etc.).

In short, there must be someone who measures the variable, who executes the logic and who acts to bring the process to a safe state.

SIL

(Safety Integrity Level)

It is a discrete value from 1 to 4. SIL defines the safety integrity requirements that must be met by each SIF executed by the SIS (IEC 61511-1: 2016 – 3.2.69).

Security integrity comprises hardware security integrity and systematic security integrity. The value 1 to 4 defines the order of magnitude of the risk reduction provided by the SIF.

Safety Integrity Level (SIL)
  • Level 4 has the highest level of security integrity and indicates a risk reduction of the order of 4 zeros (more than 10,000);
  • Level 1 security integrity has the lowest and indicates a risk reduction of the order of 1 zero (more than 10).

What does “SIL” mean?

Let us first by understanding the safety Integrity Level (SIL) that it is always associated exclusively with a Safety Instrumented Function (SIF). So, if we start from the fact that a Safety Instrumented System (SIS) is made up of different SIFs. We can understand that in a SIS there will be as many SIL‘s as SIF’s coexist.

Trying to assign a SIL to an entire plant or installation would be a conceptual error. It would also be to think that a controller (for example, one capable of achieving SIL 3), is the one that determines the SIL of a SIF or worse still of a full SIS.

This leads us to specify that SIL is a specific measure of performance related to safety and this is determined for a specific SIF. Therefore, by the time a SIF is put on demand (its operation is necessary), its behavior, from the point of view of its success or failure, will be escalated at 4 different levels according to the IEC-61511 standard.

However, a SIF with SIL 4 will have a probability of failure between 0.00001 and 0.0001 of the time, which is obviously much lower.

The Risk Reduction Factor (which is the inverse of the PFD) avoids the use of scientific notations to define the behavior of a SIF.

Returning to the example of a SIF with SIL 4, this would be interpreted in l / PFD as the fact that one in 10,000 to 100,000 would fail within a year. Safety Availability is the interpretation of the behavior of the SIF from the point of view of its success.

So then, a SIL 4 is successful 99.99% to 99.999 of the times it is required.

Implementation in risk reduction

First of all, we must bear in mind that the IEC-61508 standard defines security as “free from unacceptable risks”. Therefore, absolute safety can never be achieved. In practice, Risk can only be scale down to an acceptable level.

Generally, the security methods used to reduce risk are

  • Change the process or mechanical design including plant and equipment arrangements.
  • Increase the mechanical integrity of the equipment.
  • Improve basic process control systems.
  • Increase the frequency of testing critical process components and protection elements.
  • Apply a Safety Instrumented System (SIS).
  • Install a system with equipment and systems to reduce the consequences in the event of an accident.

Safety management requires a verification that the SIS will truly give the Required Risk reduction or will meet the specified performance value (SIL) for each SIF during its operation.

Author: Abraham Moses

Types of Valves

Valve categories

Due to different variables, there cannot be a universal valve. Therefore, to meet the changing requirements of the industrial process applications, countless designs and variants have been created over the years as new materials have been developed. All valve types fall into nine categories: Plug Valves, Gate Valves, Globe Valves, Ball Valves.

Ball valves (Flanged/ Threaded Ball Valve)

They are basically modified plug valves. They are not satisfactory for throttling, they are quick to operate, easy to maintain, require no lubrication, produce a tight seal with low torque, and their pressure drop is a function of the size of the orifice.

Image Courtesy : Flow Serve

The ball valve, also known as a quick-closing valve, or ball valve. It is a very versatile type of valve because the lever only needs to be turned 90 degrees to close or open the valve quickly.

This valve is popular in the industry due to its ease of operation and small design.

It is an excellent option when fluid regulation is not required, as this type of valve operates best when fully closed or fully open. Because, when left partially open, the fluid and its pressure would cause damage to the interior of the valve with time.

Butterfly Valve (Lug/Wafer Type Butterfly Valve)

The butterfly valve is characterized by its circular shape and the disc in its center. The lever is rotated 90 degrees to open or close the disc which in turn blocks the flow of fluid.

There are different types of butterfly valves that are categorized according to their installation method, the most common is the wafer and the Lug (shown in the image).

Image Courtesy : Neles Valves

This valve is a good option for applications that do not require to withstand a lot of pressure and do not need regulation Because the butterfly valve also operates fully open or closed, it is an excellent option to be installed in places with little space, or in process lines that do not support too much weight. 

They work at pressures of 150 psi to vacuum. Valves that do not allow reverse flow (check) act automatically on pressure changes to avoid reversal of flow, such as the check valve, for example.

They are one of the most common and oldest known types. They are simple, lightweight, and inexpensive, and the cost to maintain is also low because they have a minimum of moving parts.

The primary use of butterfly valves is for shutoff and throttle service when handling large volumes of gases, and liquids at relatively low pressures.

Needle Valve

The needle valve is the most accurate of the valves found in this article. They are basically globe valves that have needle-like conical taps that fit precisely into their seats. They are generally used for instruments or hydraulic systems.

Image Courtesy : Swagelok Valves

This type of valve is characterized by having a fine operating mechanism since, when the handwheel is turned, a needle-shaped stem descends that gradually regulates the fluid with fine and precise throttling.

This valve is useful for high-pressure applications, high temperatures, or sanitary and instrumentation requirements.

Construction materials are typically bronze, stainless steel, brass, and other alloys.

Plug Valves

The primary use of plug valves, like gate valves, is in shut-off, non-throttling service. Since the flow through the valve is smooth and uninterrupted, there is little turbulence within the valve and therefore the pressure drop is low.

Image Courtesy : DeZURIK

The main advantages of the plug valves are fast action, simple operation, minimal space for installation, and tight shut-off.

There are two main types of plug valves:

  • Lubricated type plug valves to prevent leakage between the plug surface and the seat on the body and to reduce friction during rotation. 
  • Non-lubricated plug valves have a coating that eliminates the need for lubrication.
  • Plug valve services include full opening or closing without throttling.
  • They have minimal resistance to flow.
  • They are intended for frequent operation.