Alarm Floods

02 Nov 2019 at 20:46 // under Simple Saturdays Process Control

Alarms are the control system's was of shouting at the operator. If saying "STOP WHAT YOU ARE DOING AND LOOK AT THIS". The problem is that it takes time for the operator to stop what they were doing, look at the alarm. Then look at what else is going on, understand what the problem is and decide what to do about it. If more alarms arrive during this time, then the operator has to stop looking at the current alarm or the new alarms will have to wait.

An alarm flood is when too many alarms come in for the operator to deal with. This is considered as more than 10 alarms in 10 minutes.

What causes a flood?

Consider a simple feed system.

• A tank that contains the feed.
• A pump that pumps the feed from the feed tank to the rest of the process
• A flow meter and control valve to control the rate at which the feed is delivered to the unit.

Even with this simple setup, there are a number of places where having an alarm would be useful:

• Alarm in high and low levels in the feed tank.
• Alarm when the pump stops so the operator knows if it has tripped or is someone hits the stop button.
• Alarm if the flow measurement deviates from the controller setpoint.
• Alarm on a minimum of maximum flow rate.

Now let's imagine a situation when the feed tank runs out of feed. The operator would get an initial low level alarm in the tank. This gives the operator some time to work out what is going on and perhaps fix the problem. But if the operator doesn’t manage to refill the tank or safely shutdown the unit, the level may get low enough to trigger a trip.

When a low level in the feed tank is measured, the system automatically turns the pumps off. This trip is to protect the pumps from damage by making sure that they do not run without any feed in them. This will trigger an alarm to the operator to let them know what happened.

But remember what other alarms are in the system,

• The pump stopped alarm would also trigger
• The flow deviation alarm would also get triggered as the flowrate drops off.
• The low flow alarm would also get triggered.

As soon as the trip is activated, the operator should be hit with four different alarms. And this is a very simple setup. In reality there would be others. When the flows stop, temperatures and pressures downstream would change and also trigger alarms. The level in any downstream tank would start to drop and would trigger an alarm. Other trips would probably get activated causing another series of alarms.

It is very easy to end up with hundreds of alarms within a 10 minute period all triggered by an initial trip.

This can make the operators job very difficult. How can they tell what the initial problem was when it is now buried at the bottom of many pages of alarms. Which of the many alarms that the system is telling them about should they actually be worrying and doing something about?

What can you do about it?

It is a very difficult problem to solve. One of the ways to help reduce the flood is to build logic into the control system to turn off alarms when they are not important. In the case of the example given earlier, when a low level trip is activated, the pump is expected to stop, the flow is expected to stop. In these cases, the pump stopped alarm and the low flow alarm are not important. The system could be set up to disable those alarms when the trip is activated.

This needs to be carried out for every trip. Look at what happens when the trip is activated. What alarms are expected to occur as a direct result of the trip. The other thing to consider is when to turn the alarms back on. If you do it when the reset button is pressed, then you will flood the operator with alarms at that time instead. This is not much better. Instead you want to leave the alarms disabled until they become useful again. Wait till the pump is running and enough time has passed for the flow to have started before turning the low flow or the pump alarms back on.

Trying to fix it with a single project and setup is also unlikely to work. After every trip or flood event, the team should look at what happened, which alarms were activated, which ones were actually useful and which were a nuisance. Then work out what further changes can be made to avoid the nuisance ones the next time.

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