Tank drain valve for polyethylene storage tanks (thermoplastic bottom-outlet valve)

01

Choose the outlet fitting the drain valve is mounted on

Poly Processing, a polyethylene tank maker, describes its Integrally Molded Flanged Outlet (IMFO) as a flange molded from the same material as the tank during manufacture rather than an insert, located at the bottom of the sidewall below the tank's knuckle radius so that the tank discharges fully without entry for cleaning, with all flange hardware accessible from outside. It lists the vertical IMFO tank from 200 to 15,500 gallons and the sloped-bottom IMFO tank from 3,950 to 15,000 gallons. [1]

For bulkhead fittings, the same maker warns that the fitting creeps over time so that the nut loosens, that it must be installed from inside the tank, and that it should not be used on the bottom sidewall of tanks greater than 3,000 gallons or taller than 6 feet; it lists them in PVC, CPVC and polypropylene from 1/2 in to 6 in. Its bolted flange fittings, in PVC and CPVC, are maintained from outside the tank, with socketed sizes from 1 in to 6 in and threaded sizes from 1 in to 4 in, and polyethylene-encapsulated bolt heads. [2]

ASTM D1998 covers flat-bottom, upright, one-piece rotationally molded polyethylene tanks from 1,900 L (500 gal) up for use at atmospheric pressure, with continuous service temperatures below 66 °C (150 °F) for Type I tanks and below 60 °C (140 °F) for Type II tanks. [5] Editorial note: the drain valve and its fitting see the tank's liquid at those temperatures, so state the maximum liquid temperature with the valve enquiry.

Give the supplier the tank volume, height and material, the outlet type and size (molded flange, bolted flange or bulkhead), the liquid and its concentration, maximum liquid temperature, valve type and operation (lever, gear or actuator), pipe connection (socket, thread, flange), gasket and bolt materials, and how the outlet piping is supported.

02

Pick the valve type, body and seal materials and check the rating

For the drain of its molded flanged outlet, Poly Processing offers butterfly valves from 2 in to 6 in with PVC, CPVC or polypropylene bodies, EPDM, Viton or Viton GF seals, and 316 stainless steel, titanium, C-276 or Alloy 400 bolts; its ball valves cover 1/2 in to 6 in in the same body materials with socketed, threaded or true-union connections and EPDM, Viton or Teflon seals. [3]

Hayward Flow Control lists its BYV series thermoplastic butterfly valves with injection-molded PVC, CPVC or glass-filled polypropylene bodies, PVC, CPVC, GFPP or PVDF discs, Viton, EPDM or nitrile liners and a one-piece 316 stainless steel stem. It rates 2–8 in sizes at 150 psi (10 bar) and 10–12 in sizes at 100 psi (8 bar), both at 70 °F (21 °C) non-shock, gives a Cv of 92 for the 2 in PVC valve, and states an ISO 5211 top flange for actuators and ANSI B16.10 / ISO 5752 narrow face-to-face dimensions. Its material classes include PVC cell class 12454 and CPVC cell class 23447 per ASTM D1784. [4]

Editorial calculation: water standing 1.83 m (6 ft) deep exerts about 18 kPa (0.18 bar) at the tank bottom (1,000 kg/m³ × 9.81 m/s² × 1.83 m), about 1.8% of a 10 bar rating at 21 °C (0.18 ÷ 10). Static head on an atmospheric tank is therefore small compared with the valve's room-temperature rating; ask the supplier for the rating at your liquid temperature and for chemical compatibility of body, disc and seal, which the room-temperature figure does not cover.

03

Keep the valve and piping from loading the tank wall

Poly Processing supplies flexible hose connections, made from ultra-high-molecular-weight hose with barbed nipples and stainless steel bands, to isolate the tank from the stresses of pumps and piping, in sizes from 1 in to 3 in with PVC or stainless steel nipples. Its PTFE expansion joints compensate for misalignment, expansion and contraction and isolate vibration, but the maker states they should not be attached directly to the tank wall or the molded outlet; a full-face flange must be installed on the tank first. [3]

For its double-wall tanks, the maker describes a bellows transition fitting with an expansion joint so that the tank expands freely during loading and unloading and damage from pump-induced piping vibration is avoided. [2] Editorial note: a drain valve hung on a bottom outlet carries the weight of the valve, any actuator and the outlet pipe; ask whether the outlet piping needs its own support and a flexible connection before the first fixed point.

04

From tank outlet to flexible connection: the parts of a drain assembly

05

Rating at liquid temperature and seal material come first in a drain valve order

06

Send the tank, liquid and valve data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Tank: [material; volume; height; outlet type and size]
Liquid: [chemical; concentration; maximum temperature]
Valve: [butterfly or ball; size; manual or actuated]
Materials: [body; disc or ball; seals; gasket; bolts]
Connection: [socket, thread, flange; standard]
Piping: [flexible connection; support]
Documents: [pressure–temperature rating; chemical resistance data]

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