OCEANCO Ltd

Dispelling the mystery of tank band design, selection and installation.
A set of tank bands is not something you should have to replace every few years. The next time you are shopping for double bands should be the last time.

Common Misconceptions

"Manifolds never fail" - Although catastrophic manifold failures during a dive are extremely rare, damage to manifolds from poorly constructed or improperly installed tank bands is common and normally detected during breakdown or reassembly following hydro or visuals. Hairline cracks, fractures, bent crossovers, extruded O-rings and swaged or mauled internal components are potentially dangerous manifold failures and can be attributed directly to improperly installed or poorly manufactured bands.

"Squeezing the bolts between the tanks is okay" - Your band bolts serve two purposes, neither of which is to act as a spacer between the cylinders. First and most importantly, the bolts are designed to secure the cylinders in the bands without touching the cylinder walls. Properly designed and engineered bands will not use the bolt to draw the tanks together against itself, as a spacer. When tanks are filled, their diameter actually increases slightly. With time this results in wear if the bolts are contacting the tanks.

"One size fits all" - A properly constructed double tank band is not only diameter-specific to a certain sized cylinder, but also designed to accommodate and support a specific manifold. The combination of your tanks together with your manifold, must be considered when selecting bands. Avoid manufacturers and dealers who promise their bands will fit a multitude of tanks and manifolds.

"Stainless is stainless" - Not necessarily true. Selecting a band constructed from stainless alloy 316L is highly recommended, especially if most of your diving is in the ocean. The molybdenum component in 316 results in a stainless steel that is impervious to the highly corrosive effects of sea water. Other common alloys of stainless will rust and deteriorate in a short time.
Weld Close-up
"Spot welds are okay" - Incredible shear stresses exist at the location where some bands are spot welded together. Spot welds will rust, tear and elongate with repeated fillings, to the point where the two halves separate. Your bands are not a piece of gear you should have to replace every few years. The preferred weld is a continuous fusion weld in an inert gas atmosphere followed by a surface passivation to minimize a condition known as carbide precipitation.

"My manifold is a handle" - Manifolds are designed to withstand thousands of pounds of pressure from within, but should never be subjected to external mechanical stresses. Improperly fitted or poorly constructed tanks bands can cause stresses like bending and torsional loading. This precaution also includes using the crossover as a handle for lifting. Lift by the valve posts.

"All tanks are round" - Not true. Steel tanks in particular are almost never round, nor are two of the same series exactly the same height. Surprising but true! Because of the differences in manufacturing processes though, aluminum cylinders are consistently rounder and vary little in diameter and height.

Upgrading and keeping the same bands - In some cases, this is fine. However, when upgrading to significantly taller of heavier cylinders, the bands should be proportionally wider in order to protect the manifold from mechanical loading.

Concerns during installation of your bands . . .

CAUTION! - Special Note: Never loosen the bands on manifolded cylinders that are pressurized. If your tanks are already set up and you intend to change the bands, it is critically important to purge all the gas before loosening the band bolts. Never handle manifolded, pressurized tanks without bands being securely in place.

Compatibility - Make sure the bands you select are sized correctly for the diameter of your tanks and will maintain the exact center spacing recommended by the manufacturer of your manifold. The width of the band should be proportional to the height of the cylinders. 8" tanks with the mass and height of the LP120's and taller, require the stability provided by a pair of 3" wide bands. The tanks you choose to double-up should be the same height. Steel cylinders in the same series can vary considerably in height.
Micrometer on bands

Spacing Between Tanks - A properly engineered set of bands will maintain critical cylinder spacing without the need for spacers between tanks. This includes the bolts. If when tightening band bolts, the cylinders continue to be drawn together to the point where there is bolt contact against the cylinder walls, the bands are poorly engineered or incompatible with your tanks. Tanks expand in diameter when being filled and even more so during rapid, hot fills. Bolt contact with cylinder walls will cause serious wear in time, regardless of whether the bolt is covered with a plastic sleeve.

Caliper on bolt Bolt Length - If you select a band that is supplied with a piece of threaded rod in place of a bolt, bolt length is not a concern. If your bands are supplied with real bolts though, ensure that they are sufficient length to accommodate the thickness your backplate, BC, washer and wingnut without protruding enough to damage your drysuit.

Why Bolts . . . Not Threaded Rod? - The tensile strength of a bolt is greater than the same sized threaded rod. Additionally, most stainless steel threaded rod is a higher carbon, more machinable alloy which is also more prone to rusting in sea water. A short length of threaded rod is a fraction of the cost of bolts, which is why band manufacturers provide threaded rod rather than bolts.

Ruler on manifoldCentering the Crossover - When installing adjustable manifolds, it is critically important to center the crossover member between the posts. This is easily done by engaging the first thread on both ends simultaneously. After the threads are started, keep the two posts in the same orientation, or in other words, don't rotate one post independently of the other. Lay an accurate rule across the two valve tubes that protrude into the tank necks or across the centers of the burst disks. Rotate the crossover until the center-to-center dimension corresponds to the tank center-to-center measurement (provided by the bands) and within the range recommended by your manifold manufacturer. Hand tighten the jam nuts. Hold the manifold over the tank necks and confirm the alignment.

Position Bands on Cylinder - Locate the top band vertically on the tanks at the desired location and snug-up on the nuts. Slide the lower band into position as determined by the hole centers in your backplate and snug-up that nut as well.

Tanks in Same Plane - Lay the whole assembly down on a hard, flat, horizontal surface. This will ensure the two cylinders are in-plane with each other. Alternately tighten each band bolt. Do not over-tighten to the point where there is visible distortion in the reinforcing plates under the nuts. If these plates are distorted, the tank centers provided by the design of the band, may be compromised.

Parallelism - If the cylinders are not parallel, the manifold can be damaged. Before tightening the jam lock-nuts on the manifold, check for proper assembly and parallelism by rotating the isolation valve. The isolation valve in the crossover should rotate freely, about a half turn in either direction. Position the isolator as desired and tighten the jam nuts against the valve posts. Your tanks should now be parallel, top to bottom and the manifold, free of mechanical loading. If you were not able to rotate the crossover freely, there is inappropriate stress on the manifold.

With a properly installed set of double tank bands that have been carefully selected for your tank and manifold combination, you will enjoy many years of safe diving with minimal preventative maintenance. Buy your bands once. Buy them to fit. Safe diving!

    
CAUTION! - Special Note: If you find the need to move or loosen the tank bands later after the tanks have been pressurized, it is critically important to purge all the gas from your tanks before loosening the band bolts. Never loosen the bands on manifolded cylinders that are pressurized