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Theory of Operation PLU and PLU35 Hybrid Preferential Logic Manifolds

The system consists of a control unit, one bank of liquid vessel(s) (portable bulk, micro-bulk or bulk) which will serve as the primary bank (this has been hard-coded to be the left bank), a secondary bank (minimum of 3 cylinders) of high pressure cylinders and a third emergency reserve bank (minimum of 3 cylinders) of high pressure cylinders The logic gives preference to the liquid (left) bank, always reverting to the liquid (left) bank provided there is sufficient pressure. In normal operation, the left and right banks or sources alternate between “primary” and “secondary” providing an uninterrupted supply of gas to the facility. Should the primary and secondary banks both deplete or become inoperable, the system shall automatically cascade via pressure differential to a third bank of high pressure cylinders known as the ‘emergency reserve’. The Central Gas System is designed to provide a reliable uninterrupted supply of gas to the facilities medical gas pipeline system with only small fluctuations in the gas pressure (achieved by parallel line regulators) being provided to the facility and to initiate the six NFPA 99 required master alarm signals; secondary in use, secondary (right bank) low, high line pressure, low line pressure, emergency reserve in use and emergency reserve low.

The system shall control the primary and secondary banks such that the left bank will always be “In Use” or primary whenever there is sufficient pressure while the secondary (right) bank is held in reserve as the “Ready” or secondary bank by a solenoid valve. Operationally, it does not matter which bank is pressurized first, whenever the left bank pressure reported by the transducer meets the hard-coded minimum in the logic it will be recognized by the circuit board logic as the “In Use” or primary bank. If the left bank does not have sufficient pressure but the right bank does have sufficient pressure, the right bank will be established as the “In Use” or primary bank. Simultaneously, a green “In Use” LED will be illuminated on the circuit board for that bank. A typical application cycle would begin with the left bank as the primary “In Use” bank. The right bank when pressurized will be recognized by the circuit board logic as the “Ready or secondary” bank when the pressure reported by that bank pressure transducer surpasses the minimum required pressure established in the logic. Simultaneously, a yellow “Ready” LED will be illuminated on the circuit board and the solenoid valve will be energized (closed) for the right bank. Bank pressures will be monitored by the two high pressure transducers and the bank pressures are displayed (in either PSI, or Kpa or Bar) on the circuit board display. As the gas is used, the “In Use or primary” left bank depletes to a predetermined pressure in the logic at which time the secondary in use alarm dry contact will be opened – breaking the normally closed circuit to the master alarm. Simultaneously, the green “In Use” LED will be extinguished on the left bank and the red “Empty” LED will be illuminated for the left bank. Simultaneously, a green “In Use” LED will be illuminated on the circuit board for the right bank and the right bank solenoid valve will be de-energized allowing it to open. Gas will flow from the right bank providing gas to the facility. When the gas source is replenished on the “Empty” left bank and the pressure reported by that bank pressure transducer surpasses the minimum required pressure established in the logic, the circuit board will upgrade the status of the left bank from “Empty” to “In Use”. It is important to note that the only time either solenoid valve is closed or energized in a PLU or LU35 series manifolds is when the that bank is in a “Ready” or yellow condition. When the “In Use or primary” right bank depletes to a predetermined pressure in the logic the secondary in use alarm dry contact will be opened – breaking the normally closed circuit to the master alarm. Simultaneously, the green “In Use” LED will be extinguished and the red “Empty” LED will be illuminated for the right bank. At this point in this scenario, both the left and right banks have depleted. At a predetermined pressure in the logic, the ‘secondary (right bank) low’ relay dry contact will be opened – breaking the normally closed circuit to the master alarm.

In the event that both the primary and secondary banks deplete or become inoperable, the system shall automatically cascade via pressure differential (achieved by two stage high flow regulator), to a third bank of high pressure cylinders known as the ‘emergency reserve’. At a predetermined pressure in the logic, the ‘emergency reserve in use’ relay dry contact will be opened – breaking the normally closed circuit to the master alarm. As the emergency reserve pressure continues to deplete, at a predetermined pressure in the logic, the ‘emergency reserve low’ relay dry contact will be opened – breaking the normally closed circuit to the master alarm.

The low pressure transducer will continuously monitor the line or delivery pressure exiting the system. Line pressure will be displayed (*in either PSI or Kpa or Bar). If the delivery pressure drops below and/or rises above predetermined pressures in the logic, the respective line pressure low or line pressure high relay dry contact will be opened – breaking the normally closed circuit to the master alarm.

Other features of the system are the economizer circuits. A method of economizing (using rather than venting the gas) is required by NFPA 99. In the PLU & PLU35 systems the economizer circuit consists of; an orifice, a 50 psig check valve and tubing and fittings. There are economizer circuits for both banks. The plumbing can be seen running from the left & right inlet blocks into the intermediate block. In operation, the bank of vessel(s) In Use will provide gas to the manifold which is regulated by the pre-set regulator on the vessel(s). The bank of vessel(s) in Ready status will build pressure over time because the vessel is filled with liquefied gas which gradually warms and boils or vaporizes. Liquefied oxygen boils at negative 297.35◦ F. The reason the liquefied gas warms and vaporizes is the vessel insulation (consisting of a vacuum and insulation) is not a perfect insulator and heat ‘leaks’ into the inner containment chamber and warms the liquefied gas. As the liquefied gas vaporizes, the head pressure within the vessel builds (because it is a closed system). Portable bulk vessels are not designed to withstand pressures higher than a few hundred psig and therefore are equipped with a relief valve which will relieve or vent the gas when the pressure reaches the set point of the relief valve. There are different designs of vessels in the market, but the most common for use with medical grade gases are equipped with either a 235 psig or a 350 psig relief valve. So the goal in designing the economizer circuit is to allow gas to be used rather than vent as much as possible. The way our economizer works is; when the gas pressure in the Ready bank exceeds the gas pressure provided by the In Use bank by 50 psig, the economizer circuit for the reserve bank opens allowing a minimal flow (only 3 or 4 lpm) from the Ready bank to be fed into the flow of gas being provided to the facility. The flow is minimized to only 3 or 4 lpm by the orifice. So for example; if the facility is using 100 lpm, when the economizer opens 4 of the lpm will be provided by the Ready bank and 96 of the lpm will be provided by the In Use bank. If there were no economizer circuits, the gas would be vented thru the relief valves on the vessels whenever the pressure in the vessel equaled the set point of the relief valve and wasted. Normal Evaporization Rate (NER) - A typical vessel (in good condition) if left unused at room temperature will actually vent it’s entire contents in about 35 to 50 days. This explains why Ready banks are not full when the manifold switches over and the Ready bank becomes the In Use bank. This also explains the requirement for the third bank (emergency reserve) of high pressure cylinders.

Tri-Tech liquid manifold systems also incorporate exclusive economizer software. This proprietary software senses residual liquid in near empty vessels and uses the residual gas through temporary ‘blind changeover(s)’ without triggering alarms or allowing near empty vessels to be returned to a “Ready” or secondary bank status.

It is important to note that in the event of electrical power loss, both solenoid valves automatically open. The bank with the highest pressure will provide gas until it is depleted to a pressure equal to the next highest pressure bank, then the two banks will provide gas until depleted to a pressure equal to the third (lowest pressure) bank and then all three banks will provide gas until all three are completely depleted.


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