Regularly scheduled maintenance must be conducted to ensure that the gauges and valves on a nurse tank are suitable for service. The degree of hazard increases as parts get older. A visual inspection should be completed before each use.
Some nurse tank components have a service life limitation based on time. The service life cannot be extended. Use a checklist to keep an accurate inventory when parts need to be replaced. When a component is found to be defective, it must be repaired or replaced.
NURSE TANK VALVE AND GAUGE SAFETY CHECKLIST
Check the following nurse tank parts to determine if they are OK.
- Liquid withdrawal valve (5-year replacement)
- Excess flow valve (5-year replacement)
- Liquid fill valve (5-year replacement)
- Pressure gauge
- Vapor return valve (5-year replacement)
- Liquid level float gauge
- Hydrostatic relief valve
- Bleeder valves
- Pressure/safety relief valve (5-year replacement)
- Roll cage for valves/gauges
- Tank drain
If any these checklist items are not OK, they should be replaced.
The liquid withdrawal valve connects the nurse tank hose to the interior of the nurse tank for removal of the ammonia. The intake for this valve reaches nearly to the bottom of the tank so that liquid ammonia will be withdrawn until the tank is nearly empty. It contains an excess flow valve preventing anhydrous ammonia from flowing out of the tank too quickly if the nurse tank hose breaks. The excess flow valve has a five-year replacement requirement.
Excess Flow Valve
The liquid fill valve is used while filling the nurse tank from a bulk storage facility. It has an excess flow valve preventing anhydrous ammonia from flowing back out of the nurse tank if the hose breaks. This valve has a five-year replacement requirement.
The pressure gauge indicates the interior pressure of the nurse tank. This interior pressure is affected by the quantity and temperature of the anhydrous ammonia in the tank. The pressure gauge does not have a time limit to its service life, so it needs to be replaced only when it fails to function properly.
The fixed liquid level gauge is a bleeder valve with a tube extending into the tank to a depth where the tank would be 85% full of liquid.
Fixed Liquid Level Gauge
The valve should be opened partially when the tank is being filled so it will bleed off, or spurt, liquid anhydrous ammonia when the tank is 85% full. The gauge is accurate only if the tank is level. Do not continue filling the tank after the valve releases liquid anhydrous ammonia.
The vapor return valve is used while filling the nurse tank at a bulk storage facility. As liquid anhydrous ammonia is transferred into the nurse tank, the vapor in the nurse tank is transferred to the bulk storage tank. The vapor valve has an excess flow valve (which has a five-year replacement requirement). The excess flow valve prevents release of anhydrous ammonia vapors from the nurse tank if the vapor valve fails.
A hydrostatic relief valve should be used anywhere anhydrous ammonia may be trapped in any section of the liquid or vapor lines of the system.
Hydrostatic Relief Valve
It opens at 365 psi to relieve excess pressure due to liquid expansion.
The liquid level float gauge indicates the level of liquid anhydrous ammonia in the tank.
The fixed liquid level gauge should be used to determine the 85% full level when filling the tank instead of the liquid level float gauge.
The safety relief valve also is known as the pressure-relief valve. This valve is on top of the tank and prevents excessive pressure from building up in the tank caused by overfilling or expansion from heat.
The valve opens directly to the atmosphere when the tank reaches 250 to 265 psi and closes when the interior pressure drops below 250 psi. It must be replaced every five years if it has nonmetallic seats and tested every five years in lieu of replacement if it has metallic seats. Repairs can be done only by a repair organization with a valid “VR” certificate of authorization from the National Board of Boiler and Pressure Vessel Inspectors. This valve must have a rain cap in place to prevent damage from moisture and corrosion. The safety relief valve is not adjustable and should be replaced if it is inoperable.
The rotary gauge mounted on the end of an anhydrous ammonia tank, bulk tank, nurse tank or applicator tank consists of a dip tube inside the tank and a bleeder valve.
Rotary Gauge. The gauge includes a tube with a pointer at one end. The tube turns with the pointer. Next to the pointer is the dial indicator which shows the percentage of anhydrous ammonia in the tank.
This gauge is used to determine the actual level of anhydrous ammonia in the tank. It is accurate only if the tank is level. The indicator shows the liquid level in the tank when the bleeder valve is opened and the knob is rotated to the point where liquid spurts from the bleeder valve.
Back-check valves, part of the bulk storage facility, are used to control the direction of flow of anhydrous ammonia when unloading a truck or rail tank car, preventing a backflow from the bulk tank to the truck or rail tank car.
A flow in the wrong direction causes the valve to snap shut. It will open only when the direction of flow is correct.
The emergency shut-off valve is another valve not found on a nurse or applicator tank.
Emergency Shut-off Valve. The valve has a cable attachment for remote release, an operating lever, and a poppet.
It also is part of the bulk storage facility used to stop the flow of anhydrous ammonia when the operator of the facility detects an emergency situation and decides to stop the transfer of anhydrous ammonia immediately. This valve is on the discharge side of the transfer pump.
The roll cage protects gauges and valves if the nurse tank rolls over. The roll cage must be strong enough to prevent damage to the gauges and valves.