15-06-2012, 05:11 PM
Interlocking
Interlocking is a method of preventing undesired states in a state machine, which in a general sense can include any electrical, electronic, or mechanical device or system.
In most applications an interlock is a device used to help prevent a machine from harming its operator or damaging itself by stopping the machine when tripped. Household microwave ovens are equipped with interlock switches which disable the magnetron if the door is opened. Similarly household washing machines will interrupt the spin cycle when the lid is open. Interlocks also serve as important safety devices in industrial settings, where they protect employees from devices such as robots, presses, and hammers. While interlocks can be something as sophisticated as curtains of infrared beams and photodetectors, they are often just switches.
Trapped key interlocking
Trapped key interlocking is a method of ensuring safety in industrial environments by forcing the operator through a predetermined sequence using a defined selection of keys, locks and switches.
It is called “Trapped Key” as it works by releasing and trapping keys in a predetermined sequence. After the control or power has been isolated, a key is released that can be used to grant access to individual or multiple doors.
For example, to prevent access to the inside of an electric kiln, a trapped key system may be used to interlock a disconnecting switch and the kiln door. While the switch is turned on, the key is held by the interlock attached to the disconnecting switch. To open the kiln door, the switch is first opened, which releases the key. The key can then be used to unlock the kiln door. While the key is removed from the switch interlock, a plunger from the interlock mechanically prevents the switch from closing. Power cannot be re-applied to the kiln until the kiln door is locked, releasing the key, and the key is then returned to the disconnecting switch interlock. [1] A similar two-part interlock system can be used anywhere it is necessary to ensure the energy supply to a machine is interrupted before the machine is entered for adjustment or maintenance.
What is Boiler Interlock?
You should not fit a TRV to the radiator nearest to the room thermostat
(if you do already have a TRV here, then leave it on its maximum setting).
The room thermostat (sometimes called a roomstat) is usually found in the hall.
The boiler is turned off when the temperature set by the room thermostat is reached.
This is called a boiler interlock. It prevents the boiler from firing when there
is no demand for heat from the central heating system.
If all of the radiators in the house are fitted with TRVs, including the radiator nearest to the
room thermostat, then all the TRVs can close at the same time and turn off all the radiators
while the temperature in the house is still below the temperature set by the room thermostat.
In this situation the room thermostat has not turned off the boiler but all the radiators are turned off.
The boiler will keep running and waste energy. This arrangement is not allowed by part L of the
current building regulations concerning the conservation of fuel and power.
An automatic bypass valve must also be fitted.
How does a TRV work? - Thermostatic Radiator Valves.
Thermostatic radiator valves (also called TRVs)
allow you to control the temperature of each room independently.
They are fitted to each individual radiator and are a very energy efficient way
of controlling a central heating system (sometimes called space heating).
The TRV must have a free flow of air around it. Try to avoid covering it with curtains.
Without TRVs, all the radiators in the property will try to raise all of the rooms
to the temperature which has been set by the main room thermostat.
The room thermostat (sometimes called a roomstat) is usually found in the hall.
Without TRVs we can still turn off an individual radiator completely at the valve
but this will leave the room with no heating.
With TRVs, each radiator can be set to control the temperature of the room.
This allows some rooms of the house to be warmer than others, or unheated if you wish.
It is easy to change the temperature setting of the TRV by rotating the top of the valve.
Turning the TRV up will make the room heat up to a higher temperature but it will
not make the room heat up more quickly. The rate at which the room heats up depends
on the boiler, radiator size, room size and the amount of insulation in the roof and walls.
Turning the TRV down will maintain the room at a lower temperature
(using less energy and saving you money) but it will not make the room cool down more quickly.
The rate at which the room cools down depends only on the amount of insulation.
To fit a TRV to a radiator, the heating system needs to be drained down
and then refilled with the addition of a corrosion inhibitor.
You should not fit a TRV to the radiator nearest to the room thermostat - see boiler interlock.
Abstract:
This paper describes a very interesting industrial safety problem and its solutions.
Currently, boiler safety controls utilize pressure drop across various boiler elements in an attempt to guarantee adequate combustion air flow, purge air flow, and atomizing media flow.
This paper describes various practical situations where these flow measurements are inadequate.
Various alternative measures are described to mitigate the problems presented.
The paper should serve as a basis for complete re-thinking on these safety controls on both power, industrial, and commercial boilers.
Keywords:
boiler, safety control, flow measurement.
1 Introduction:
A typical industrial or steam boiler utilizes approximately 50 different alarms and interlocks to satisfy safety for automatic operation.
Some of these interlocks require that flow rate be measured and compared to a minimum requirement. There are two kinds of interlocks that are used in the boiler safety control system: 1.
pre-ignition or start-up interlocks and 2.
running interlocks. The start-up interlock monitors the pertinent parameter only during start-up and once that parameter is satisfied it allows the boiler to continue in the start-up sequence.
Running interlocks are active once the boiler begins firing.
These interlocks use a continuously measured signal to shut the boiler down if the pertinent measured parameter does not meet its requirement.
A running interlock interrupts boiler operation by causing the closure of two automatic fuel shut off valves.
A pre-ignition interlock causes the safety control system to return the boiler to its initial start-up position.
Three of these controls are considered in this paper.
For each of these controls, the system is described, the problems are ...