Features of a control system for enterprises with potentially explosive (1.14, с. 103)

Features of a control system for enterprises with potentially explosive (1.14, с. 103)

Basics of explosion protection

 

Often, DCS, introduced in the mining, chemical, petrochemical and gas industries are established in areas of production that are characterized by potentially explosive consequence of any permanent presence of an explosive environment, or a high probability of occurrence of such an environment in case of accident or violation of the flow process. Failure to comply with the rules of explosion can cause as many casualties as well as irreversible damage to the environment. That is why the use of highly reliable and secure technical solutions for explosion protection is one of the biggest challenges facing designers APCS.

The emergence of the risk of explosion cause the following adverse conditions:

1. The presence of flammable vapors, liquids, gases or combustible dust.

2. The presence of oxidizer - air or oxygen.

3. Formation energy of ignition - electrical or thermal.

To provoke the explosion must have the above components in certain proportions. Thus, in order to blast air / gas mixture must contain an oxidizer in a certain range of concentration. In this explosive mixture should be in contact with the body, which could send him enough for ignition energy (for example, with strongly heated conductor or sparkling contact).

All known and practiced methods of protection are aimed at reducing the risk of explosion to an acceptable level. Thus, if the system is designed correctly, single fault in any of its components should not lead to an explosion.

Methods of explosion

In general, all methods of explosion protection can be divided into four main groups.

A. Reducing the likelihood of an electric spark.

By this method, the following types of protection are:

1. Explosion protection type "e" (increased safety) provides additional structural measures against the possibility of exceeding the permissible temperature and the occurrence of arc and spark discharges, which during normal operation does not occur.

2. Explosion protection type "n" provides additional structural measures against the possibility of exceeding the permissible temperature and the occurrence of arc and spark discharges in normal and some abnormal operating conditions.

3. Explosion protection type "s" (special) can be achieved through: the conclusion of the electrical circuits in a sealed envelope with the degree of protection IR67; electrical sealing material with insulating properties (compound, sealants); impact on the explosive mix of devices and substances to absorb or reduce the concentration of the latter.

B. Isolation of electrical circuits of explosive mixtures.

The method assumes the conclusion of electrical circuits in a special envelope, filled with gaseous, liquid or solid dielectric so that the explosive mixture was not in contact with electrical circuits.

By this method, the following types of explosion:

1. Explosion protection type "m" - fill by a special compound.

2. Explosion protection type "o" - an oil filling of the shell.

3. Explosion protection type "a" - filling the shell with quartz sand.

4. Explosion type p "- fill or blowing a shell explosion-proof gas under excess pressure.

C. Holding of the explosion.

Under this method is implemented explosion protection type "d" (blast shell).

It is understood that the electrical circuits are placed in a special cased with a small gap. This does not prevent contact of electrical circuits with an explosive mixture and the possibility of ignition, but it is guaranteed that the shell constrains resulting from the blast overpressure, ie flash does not go beyond the flameproof enclosure.

As the hot gases have different penetrating ability, it is taken into account the subgroup of gases.

D. Limiting the power of the spark.

Under this method is implemented protection type "i" (intrinsically safe circuit). This method assumes that in the event of a spark of its power will be insufficient to ignite an explosive mixture. However, this method does not preclude contact explosive mixture with electric circuits.

Due to its versatility, safety and ease of implementation, the type of protection "intrinsically safe circuit" (IS, intrinsically safe circuit) is most often applied in the construction of CAM, and therefore more speech will focus on it.

Application

Consider an example. By the process control system connected transducers. This converter is located in hazardous areas. However, engineers did not take into account this fact and designed the system for normal conditions. Possible consequences of such misuse of the technical solution shown in Fig. 1 - 45:

Spark Safety Barriers

In the minimum configuration is a barrier terminal blocks, which connect two electrical circuits. One chain forms the so-called spark dangerous (unprotected) segment, the other - intrinsically safe (protected). By unprotected segments comprise an electric line connecting the barrier with the corresponding input or output I / O modules. Intrinsically segment runs directly through the volatile area and connects the barrier with the field device. It is extremely important to remember that although the barrier and provide flame proof chain connected to it, it is not a safety explosion device and its installation in hazardous areas under any circumstances is unacceptable.

Typical diagram of the connection of barrier is shown in Fig. 1 - 47.

 

Spark safety remote I / O

In the previous section we discussed the construction of intrinsically safe I / O with the use of barriers. However, often such a linear approach is not justified. Situation will be clarified following illustrative example: let's designed control system with the number of signal input / output (1000 EX IOs), which must be intrinsic safety, reaches thousands. Consider what equipment it would require. First, we must organize itself I / O to 1000 channels. Secondly, it is necessary to ensure the intrinsic safety field signals by means of suitable barriers. At an average price of a barrier in the 160 Euro (This is the average price barriers for analog and digital signals) of their total value will be 160 Euro x 1000 = 160 000 euros. Moreover, this price does not include additional sources of supply, cables, terminal devices and related mounting part. In view of all listed, the cost of solutions for spark protection could rise to 180 000 - 190 000 euros. This very impressive amount is cause for reflection.

Fortunately, there are more elegant, and most importantly - cheaper, solution. Many manufacturers APCS combined subsystem remote input / output and standard of intrinsic safety barriers in a single modular device, and gave him a complex called "node intrinsically safe remote I / O" (Intrinsically Safe Remote IO, or abbreviated - IS RIO). The peculiarity of this device is that INDICATORS connected circuits implemented in field I / O modules, and the device is designed for installation directly in hazardous areas (Ex-zones).

As an example, in Fig. 1 - 50 shows the node IS RIO ET200iSP series produced by Siemens. Nodes ET200iSP allow installation in Ex-Zones 1, 2 and serve to connect sensors and actuating devices operating in Ex-zones 0, 1 and 2.

Variable Process

Under the phrase "process variable" refers to the numerical parameters defining the current state of the process. For process-related variables include the signals input / output parameters of the functional blocks, local and global flags (variables), tags, SCADA, etc.

Process variables are divided into discrete and analog. A discrete variable can take a finite number of values from a fairly narrow range. In practice, a discrete variable usually involve the value of a Boolean type (binary), indicating one of two possible states of the object (or control signal), although, formally speaking, it is not entirely correct. In the general case of a discrete variable is similar to the type of enumeration language C.

Analog variable can take any value from the limited continuous range. By type representation of the analog variable is the real number.

How Process variables are written to the archive? There are two techniques of their registration in the archive:

1. Cyclical record (cyclic archiving) implies a periodic record of the current value of process variable via user-defined intervals, regardless of the value and rate of change of this variable (see Fig. 1 - 54). Although this technique is not very economical, it is often used for archiving of analog variables. Frequency loop recording for each variable can be adjusted individually and as a rule, lies in the range from 0.5 to 10 min. As for discrete variables, and the rapidly changing analog variables, such an approach to the archive records clearly not optimal.

Fig. 1 - 54. Cyclical recording of variable process in the archive

2. Backing to change the variable (delta-archiving). This approach involves recording the variable in the archive only when a change in its value, compared with the previous recorded (absolute difference), reaches a certain value - delta (Fig. 1 - 55). The value of the delta set by the user and can be expressed in absolute units, and as a percentage of the scale. Of course, this technique is more economical than the cyclical record, as it adapts to the rate of change of archived values. For discrete variables - this approach is indispensable. Suppose we have a discrete variable, which varies, say, once per hour. Why did it back up every second or minute? It is much more logical to write down the value of the archive only in those moments when the value goes from 1 to 0 or vice versa.

Alarm

Emergency alarm system (alarm) - this alert the operator of a certain event involving a violation or threatened violation of procedural flow process.

Alarm configured by setting limits (boundaries, thresholds) individually for each of process variable. The system automatically tracks the change of process variable and compare its value with a pre-configured limits. In case of a variable for the normal boundaries, the system generates an alert and fixes it in the magazine alarm. Consider the most frequently used alarm for analog values:

Lo - lower warning limit. In the case of process variable is less than Lo, generated a warning alert.

LoLo - the lower boundary of an emergency. In the case of process variable is less LoLo, generated alarm.

Hi - the upper warning limit. In the case of process variable becomes more Hi, generates a warning alert.

HiHi - the upper limit of emergency. In the case of process variable becomes more HiHi, generated alarm.

DEV_HI (DEVIATION_HI) - upper limit of deviation (mismatch). If the difference (absolute value) between the two variables becomes more DEV_HI, is generated alarm. For example, such an alarm can be set up in block PID. In this case, the system will signal the rejection of the controlled quantity of the set value, exceeding the limit DEV_HI. By analogy, you can configure alarm DEV_LO.

ROC_HI (RATE_OF_CHANGE_HI) - upper limit of the rate of change. The system monitors the rate of change of process variable (first derivative). If the rate of increase in the variable above the ROC_HI, is generated alarm.

For discrete variables, alarms are much less. In fact, there are only two: the emergency state, the corresponding value of 1, and the accident, in case the value 0.

Fig. 1 - 57 is a diagram of the appearance of alarm on the example of a rapidly changing of process variable. It should be noted that the figure shows, not all generated alerts. For example, when returning variable back to the normal range of values than shown in the figure, an alert is generated RETURN_TO_NORMAL.

Features of a control system for enterprises with potentially explosive (1.14, с. 103)