(The following discussion is a composite of information available from manufacturers on the internet and personal experience)
Storage and transfer systems for 50% caustic must consider the both the specific nature of the liquid and the economics of operation.
Bulk storage tanks for receipt of truck or railcar delivery typically incorporate the following features:
Careful consideration should be given to locating the bulk tank. Tank bottom elevation is important.
Ideally, the best location is at the same elevation as the delivery vehicle and within 200 feet of the delivery location.
An area that is heated and has potable water and drains (for washdown) available is best.
If caustic soda is to be maintained and stored at 50% a source of heat (electrical or steam) must be also considered
Storage Tanks should be sized for 1.5 times the maximum expected delivery size. This extra capacity is required due to the swelling experienced on delivery.
For tank truck deliveries of ~4000 US gallons, a 6000-7000 US gallon tank is acceptable. For larger capacity, a 12,000 gallon tank is a good size.
For railcar deliveries,
A balance must be struck between delivery that is too frequent and the cost of continuous heating of caustic.
For most users of 50% caustic, a four to 14 day periodic delivery schedule is attractive.
MATERIALS OF CONSTRUCTION:
Bulk Storage tanks of 50% caustic can be made from mild steel (if iron “pick-up” is not a concern to the process). Stainless steel and other alloys can also be used.
Tanks of mild steel with internal/external epoxy coatings have been used in cases where iron pick-up is unacceptable.
Tanks made from mild steel must not exceed 125F to prevent caustic corrosion cracking.
Polypropylene can also be used for smaller day tanks. FRP is not recommended without PE/PVC or PTFE linings.
- Tanks should be specified, designed and constructed to API-650, API-620 or ASME Section VIII.
Because tanks in caustic soda service will have a specific gravity greater than 1.0, the UL-142 code is not acceptable.
- For 50% caustic storage, a design specific gravity of 1.6 is recommended
- Because caustic soda is difficult to pump when the temperature gets to ~60F, Storage tanks must be heated or kept in a warm room.
- Electric pad heaters set at 80F are easy to control and maintain. A shell mounted temperature gauge is useful.
- A periodic cleaning schedule of 2 to 4 years is suggested. Calcium/magnesium hydroxide and other compounds will collect as sediment inside the tank.
Shell/Roof and Bottom thickness:
- A shell and roof thickness not less than 0.25 inches is suggested.
- A bottom thickness of 0.375 inches is advised to provide margin against corrosion.
Aspect ratio and freeboard:
- The configuration that typically yields the cheapest tank is a vertical tank with a flat bottom.
Tanks that have an “aspect ratio” (shell height to diameter) of between 0.9 and 1.2 are usually cheaper.
- A freeboard of three inches is acceptable
Anchorage and foundations:
- Bulk tanks set on a pad are typically easier to maintain and keep clean.
The pad should be a minimum of six inches high and can incorporate a leak detection configuration.
- Anchor bolts are not always required on flat bottomed tanks.
They are only required where the tank is subject to sinificant wind or seismic loading ( See reference #1)
- Foundation design should consider the incorporation of a flush “clean-out nozzle” in the tank design
Nozzle design and Sizes:
- The inlet piping and nozzle should incorporate a “dip-tube” (a dip-tube is a braced, internal pipe that prevents foaming and local erosion of the tank shell)
A dip tube that runs to within 12 inches of the tank bottom. The dip tube should discharge onto a square plate welded to the tank floor)
- A “clean-out” nozzle ( not less than 4 inches NPS), flush to the bottom of the tank will facilitate maintenance and inspection. This nozzle can be either blinded or fitted with a valve and hose connection.
- A spare nozzle outlet and spare vent on the roof is a good investment
- Bulk caustic storage tanks should have a generously sized vent.
Indoor tanks should be piped so that they are vented outdoors. Use a stainless steel mesh “bird screen” on the vent to prevent blockage.
- Because caustic soda is non-flammable, no emergency vent system is required
- Bulk caustic storage tanks should have a weir type overflow that is seperate from the vent.
If possible, the overflow should flow to a sump location that is obvious to the person filling the tank.
Outlet (Pump Suction):
- The tank outlet should be no smaller than 3" NPS
- For vertical tanks, the centerline of the pump suction nozzle should be located no closer than 12 inches from the tank bottom. This is to ensure that “iron rich” caustic and any debris stays in the tank
- For horizontal tanks, the pump suction nozzle should protrude 3 inches into the tank
- Gauge glasses are not recommended due to clogging and difficulty keeping warm. Ultrasonic level detection has been widely used
- Some companies install a small gauge hatch on the tank roof for manual gauging
- A high level alarm (float switch) mounted on the tank shell at the 95% level is suggested.
- Some companies tie the level alarm to automated closure of a control valve on the inlet
- As a minimum, a roof mounted 24 inch manway is suggested. Tanks larger than 10,000 gallons should consider both a roof and shell manway.
- Consider a tank mounted ladder to the tank roof for manway/tank roof access
- Two inches of polyurethane or cellular glass is acceptable for keeping the tank below 125F
Piping systems should be seamless carbon steel, but stainless steel and other materials are acceptable, but more expensive.
The entire piping system should be sloped to be free draining.
Consider adding valved air-purge connections to promote easy cleaning
Flanges and pipe joints:
- Butt welded and socket welded joints are recommended. Flanges and uniouns should be kept to a minimum.
- Flange guards (splash shield inserts) should be used.
Valves and trim:
Flanged, quarter turn, Plug or ball valves are recommended with PTFE trim.
Ductile iron, cast iron and stainless steel have been used successfully.
Dow Chemical recommends Alloy 20
Centrifugal pumps with packing or mechanical seals are often used.
Pumps with packing will become a maintenance item as the packing will allow buildup of NaOH at the shaft.
Mag-drive pumps have also been used.
Rotary pumps are an acceptable alternative.
Pump motor sizing must consider the higher specific gravity of the liquid.
- Document the tank thickness upon installation for use later
- Ensure that all personnel protective equipment is used during offloading and maintenance.
- Safety showers,eyewash stations and potable water supply are recommended at the offload location and at transfer/metering pumps.
R E F E R E N C E S :
- API-650; “Welded Steel Tanks for Petrolum Storage”; American Petroleum Institute
- “Aboveground Storage Tanks” Phillip Myers; McGraw Hill Publishing
- “Caustic Soda” OLIN Corp: Chlor Alkali products Division
- Dow Chemical Company - various papers and guidlines
- PPG “NaOH - Caustic Soda Handbook” 1991 PPG Industries, Inc