Instrumentation: [Weekly Question] Week 18 of 2011: How to operate the instrument three-valve assembly? What should be noted?

Do not subject to unilateral pressure; if there is isolating fluid or condensate, prevent it from leaking

To turn on positive pressure, open the balance valve; to turn on negative pressure, those with isolation fluid should first close the balance valve before turning on positive and negative pressures?

It needs to be done in two steps. If the three-valve assembly is to be put into use, the sequence is as follows: close the balance valve, while opening the high-pressure valve and the low-pressure valve simultaneously (theoretically, the high-pressure valve should be opened first, then the balance valve closed, and finally the low-pressure valve opened). If the three-valve assembly is to be discontinued, the sequence is reversed.

The last edit to this post was made by denghl on 2012-12-21 at 20:56. Two principles must be observed when operating the three-valve instrument cluster: (1) It is not allowed for condensate or isolation fluid in the pressure guiding tubes to leak out； (2) The measuring element (diaphragm box or bellows) must not be subjected to pressure or heat ； The starting sequence for operating the three-valve assembly should be: (1) Open the positive pressure valve ； (2) Close the balance valve ； (3) Open the negative pressure valve. The sequence of shutdown is the reverse of the above: (1) Close the negative pressure valve ； (2) Open the balance valve ； (3) Close the positive pressure valve. Furthermore, for steam flow meters and level gauges with negative migration, it is not allowed for the positive and negative valves as well as the balance valve to be open simultaneously during the operation of starting up or shutting down the three-valve assembly, not even for a very short period of time.

This post was last edited by denghl on 2012-12-21 at 20:58: close the negative pressure valve, open the balance valve, and close the positive pressure valve (deactivate it). When in use, open the positive pressure valve, close the balance valve, and open the negative pressure valve. When an isolation fluid is present, it is necessary to ensure that the three valve assemblies are not opened simultaneously, in order to prevent the isolation fluid from escaping due to pressure differences.

This post was last edited by denghl on 2012-12-21 at 20:59. Reply to 1# Baghdad: Two principles: 1. Do not allow the condensate or isolation fluid in the pressure guiding tube to be lost； 2. The measuring element must not be subjected to pressure or heat. Startup sequence: 1. Open the positive pressure valve. 2. Close the balance valve. 3. Start the negative pressure valve. Shutdown sequence: 1. Close the negative pressure valve. 2. Open the balance valve. 3. Close the positive pressure valve

The last edit to this post was made by denghl on 2012-12-21 at 21:00. Two principles must be observed when operating the three-valve instrument cluster: (1) It is not allowed for condensate or isolation fluid in the pressure guiding pipes to leak out； (2) The measuring element (diaphragm box or bellows) must not be subjected to pressure or heat ； The starting sequence for operating the three-valve assembly should be: (1) Open the positive pressure valve ； (2) Close the balance valve ； (3) Open the negative pressure valve. The sequence of shutdown is the reverse of the above: (1) Close the negative pressure valve ； (2) Open the balance valve ； (3) Close the positive pressure valve. Furthermore, for steam flow meters and level gauges with negative migration, it is not allowed for the positive and negative valves as well as the balance valve to be open simultaneously during the operation of starting up or shutting down the three-valve assembly, not even for a very short period of time.

This post was last edited by denghl on 2012-12-21 at 21:02. The sequence for opening the three valves: (Assuming the initial state is with the positive, negative, and balance valves all closed) first open the balance valve, then open the positive pressure valve, close the balance valve again, and finally open the negative pressure valve. Closing sequence of the three-valve assembly: (Assuming the initial state is with the positive and negative valves open and the balance valve closed) First, close the negative pressure valve, then open the balance valve, and finally close the positive pressure valve. The operating principle of the three-valve assembly is to prevent the diaphragms or bellows of the measuring instruments from being subjected to pressure in only one direction, and to avoid any loss of condensed water or liquid inside the pressure guiding pipes. In practical operation, for steam flow meters and level gauges with negative migration, it is not allowed for all three valves in the three-valve assembly to be open at the same time when starting or stopping them, not even for a short period of time.

This post was last edited by denghl on 2012-12-21 at 21:03. Two principles: 1. It is not allowed for the condensed water or isolation fluid in the pressure guiding tube to be lost； 2. The measuring element must not be subjected to pressure or heat in one direction. Startup sequence: 1. Open the positive pressure valve. 2. Close the balance valve. 3. Start the negative pressure valve. Shutdown sequence: 1. Close the negative pressure valve. 2. Open the balance valve. 3. Close the positive pressure valve