A tank dewatering valve cannot be selected from the outlet flange alone. The duty depends on which liquid must leave, which must stay, how clearly they separate and what head is available as the tank empties. A short, disciplined application brief can save an expensive round of redesign.
Begin with the two liquids, not the pipe size
Identify the heavier phase to be drained and the lighter phase to be retained. Include likely changes in composition across deliveries or operating modes. A description such as “oil and water” leaves too much unsaid if the site handles brine, crude blends, additives, wax or solids.
Density is especially important for a mechanically responsive decantation valve. Provide specific gravity for both phases at the minimum, normal and maximum temperatures. A stated density difference at room temperature is not enough if the system starts cold or runs hot. Also say whether the process actually allows a distinct interface to form before the liquid reaches the valve.
Build a realistic hydraulic envelope
For gravity drainage, show the liquid head at the start and near the end of a cycle. Include downstream back pressure, pipe length, bends, restrictions and the destination. If pumping or a pressurised vessel is involved, the assessment needs a fuller control and pressure picture. Catalogue capacity at a stated test condition is not a promise of site flow.
State the water accumulation rate and a sensible target drain time. The receiving system may impose its own limit. Choosing the largest nominal valve in pursuit of a fast drain can miss the more important question of reliable response across the normal operating range.
Include the conditions that affect movement and maintenance
Viscosity changes with temperature and can affect flow and the behaviour of moving parts. Solids, scale, wax and sticky deposits deserve explicit mention, even if their concentration seems modest. Include access for inspection and any flushing or isolation arrangements; “automatic” does not mean maintenance-free.
The Belfield Valve selection guide sets out the full nine-input enquiry, including temperature, viscosity, solids and a marked-up installation drawing. It also explains why a cleanly separated interface is an application requirement, not a detail to assume.
Connect the valve duty to interface information
The valve and an interface sensor solve related but different problems. A valve acts on the separated liquids and available hydraulics; an instrument may provide a local composition or boundary indication to the wider control system. Monitoring hydrocarbon in a discharge stream is a third question; PMA News discusses selecting an oil-in-water monitor separately. Defining these duties together can make the operating assumptions clearer.
A good supplier response should state the proposed model and materials, the capacity basis, the interface and liquid-property assumptions, installation requirements and open points. If a blend, temperature or pipe route later changes, the selection basis can then be revisited instead of guessed.
For a reviewed tank-dewatering application, send a process sketch and operating envelope to PMA. The Belfield Valve site has more detail on the mechanical decantation approach and the information needed for an enquiry.