DCS (Distributed Computer System)
Manufacturing plants use computer systems called a DCS, which stands for Distributed Computer System. The DCS is the “brain” behind the process. The DCS gathers inputs from automated equipment in the plant.
An automated equipment has a small sensor or transmitter that sends signals back to the DCS continuously. Equipment such as pumps have a sensor built in that tells the DCS how fast the pump is spinning (or rotating). Such a sensor is called a speed sensor.
Certain transmitters also have this signaling mechanism built in, including:
- Level transmitters
- Pressure transmitters
- Temperature transmitters
Figure 1 below gives a simplified depiction of the three types of transmitters commonly found on vessels and reactors – level, pressure and temperature transmitters.
As the name suggests, level transmitters measure the liquid level inside a tank or reactor.
Pressure transmitters measure the pressure inside the tank or reactor. A pressure transmitter can also measure the discharge pressure of a pump, i.e. the pressure of the liquid that is being pushed out of the pump.
Temperature transmitters measure how hot or cold the contents of the tank or reactor are.
Knowing how your equipment is operating is critical. Using these transmitters gives you an “inside” look into your equipment.
Most transmitters come pre-configured to take readings in real time. Sometimes, however, the plant needs to make small changes to the transmitter settings based on process conditions.
The Pressure – Temperature Relationship
Pressure transmitter readings are affected by changes in temperature.
Think of a pot of stew on the stove as shown in Figure 2 below.. If you cover the pot, you notice that the stew heats up much faster than if you left it uncovered. Covering the pot locks in the boiling contents of the stew. With nowhere to escape, the pressure inside the covered pot starts to increase. An increase in internal pressure results in a corresponding increase in temperature inside the pot of stew.
Manufacturing plants use this simple but powerful principle to operate reactors at high temperatures. A reactor can be set to a high temperature without having to heat up the reactor excessively. Heating a reactor can be expensive since it requires core utilities such as steam.
Using pressure to your advantage
The reactor is first filled with the desired components, called reactants. Once the reactor has been filled to a preset level, inert gas such as nitrogen is sent to the reactor. All exit valves on the reactor are closed at this time.
Just like with the pot of stew, the internal reactor pressure starts to increase with nowhere for the pressure to go. As the pressure starts to increase, the internal reactor temperature starts to climb.
Once the target temperature is met, the inert gas is turned off and the reaction continues.
Side Note on Process Control
The inert gas is typically fed to the reactor via a control valve. A control valve is a type of valve that you can open or close depending on an external setpoint. If the target reactor pressure starts to decrease, the control valve opens up to allow more inert gas into the reactor. If the target pressure starts to increase, the control valve closes off to prevent inert gas from entering the reactor.
In this example, the flow of inert gas into the reactor is being controlled by pressure. This is an example of a type of process control based on pressure.
The Wrong Sign
At one of the manufacturing plants where I worked, there was a reactor with 2 incoming feed chemicals. This reactor was run under high pressure to increase the temperature of the reactor. Increasing the temperature increases the rate of reaction. The faster you can make a product, the faster you can get it out the door and to your customers.
There was an equation used to calculate the temperature of the second feed chemical based on pressure. This equation was important, since this chemical was unstable above certain temperatures. This equation had been developed many years before when the plant was first built.
Over time, the operators started to notice that they could no longer rely on the DCS to automatically adjust the pressure and temperature to stay within the set point. They had been forced to independently monitor the pressure and temperature themselves. They then had to manually make changes to bring the pressure and temperature back to setpoint
This was a very large and complex plant. As you can imagine, the need for manual adjustment ended up being a sore point for the operators.
I had a problem to solve. Why was the DCS no longer able to automatically respond to changes in pressure and temperature like it used to?
I started to dig. After a few days of sifting through the original design documents, I discovered a negative sign in the wrong place in the equation. A little old negative sign. That was the problem. The wrong sign. Imagine that huh!
When I brought up the negative sign to one of the design engineers, he let out an embarrassed chuckle. As complex as the plant was, it would be a little thing that caused such a big issue. As they say, the devil is in the details. Indeed.
I worked with the operators to update the equation in the DCS. And just like that years of frustration evaporated.
What struck everyone was how quickly I had been able to find this issue. I looked at them and said, proper documentation of course!
Effective Documentation Starts With You
How many hours of precious engineering time can you save with effective documentation?
Do you really need your best and brightest sifting through mountains of paperwork that don’t make sense?
Or would you rather them focusing on troubleshooting and continuous improvement?
Every hour spent counts.
Every dollar saved counts.
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To learn more about how we can partner to develop effective documentation for your manufacturing facility, reach out today.