Article written by: Jason Danaher Ph.D.
Bulkhead fittings are designed for use as inlets or outlets on tanks, aquariums, and sumps. It provides a leak-proof way of attaching PVC plumbing to the tank or vessel. Bulkhead fittings for aquaculture are commonly manufactured from either polypropylene or polyvinyl chloride (PVC) and typically the unit is comprised of a threaded body with flange, silicone gasket, and locking nut to ensure a watertight seal for easy, long-lasting installation (Figure 1).
Figure 1. Components of a bulkhead fitting include the flange, threaded body, gasket and nut.
To install a bulkhead fitting you will first need to determine the hole size required for the bulkhead fitting. Standard drill bits will be too small of a diameter for pipe sizes used in aquatic systems. Typically hole saws are used for cutting the appropriately sized opening in the tank for the bulkhead fitting. Some bulkhead fitting manufacturers recommend the appropriate hole saw diameter based on the dimensions of the unit you purchased. The information either arrives with the bulkhead shipment or can be found on the vendor’s website. If not, you can determine a suitable hole saw size diameter by measuring the outside diameter of the threads on the body of the bulkhead (Figure 1).
It is important to note that a 2-inch bulkhead fitting cannot be properly installed with a 2-inch hole saw. The actual diameter of the bulkhead body, including the threads, will be greater than 2-inches. The 2-inches designated for the bulkhead fitting is the diameter of the inlet and outlet of the bulkhead where it will be joined with 2-inch PVC pipe or fittings.
The 2-inch bulkhead fitting in Figure 2 was measured and determined to have a 3-inch thread diameter. A 3-inch hole saw is appropriately sized for the bulkhead unit as the teeth on the hole saw are the same diameter as the threads of the bulkhead fitting’s body (Figure 2). In the process of cutting the opening into the tank wall, the hole saw will create an orifice slightly wider than the threaded body of the bulkhead (Figure 1). You should not use a hole saw with a diameter wider than the threaded body of the bulkhead fitting as it could result in a leak where either the flange, gasket, or nut cannot create a water tight seal because of the enlarged hole. In addition, an oversized hole may allow the threaded body to shift over time under pressurized conditions resulting in a leak.
Figure 2. Ensuring the 3-inch hole saw diameter is correct for the bulkhead fitting (left) by mating the hole saw to the threaded body of the bulkhead fitting (right).
With the appropriate hole saw identified, you will want to mark the location on the tank where the center of the bulkhead fitting will be located (Figure 3). In this example we have a fiberglass tank with dimensions of 96-inches x 48-inches (length x width) and are installing the bulkhead fitting for a bottom drain. The center of the drain will be plumbed approximately 24-inches off the side wall and 8-inches off the end wall (Figure 3).
Figure 3. Marking the center of where the hole saw will penetrate the rectangular tank for a bottom drain using the bulkhead fitting.
We are ready to drill the hole after the center of the bulkhead fitting is marked on the tank bottom. Most drills have a direction switch near the trigger or on the housing of the drill. The switch is typically labeled with arrows pointing in opposite directions, indicating the forward (clockwise rotation) and reverse (counterclockwise rotation) directions. For the following steps you will want to identify the switch and be prepared to swap from the reverse to forward direction as needed.
We can now align the drill bit of the hole saw on the centerline marking for the bulkhead fitting and begin to make a pilot hole (Figure 4) with the direction switch set in the forward position. Stop drilling when the drill bit penetrates through the tank bottom and the hole saw teeth make contact with the gel coat on the tank bottom. Next, set the direction switch in the reverse direction and slowly begin drilling the hole while the hole saw rotates counterclockwise. Operating the hole saw in reverse for several rotations will create a shallow surface cut in the fiberglass and create a grove for the hole saw teeth to follow when operating in the forward position. Then, change the direction switch of the hole saw to the forward position and begin operating the drill in a clockwise rotation. Start with moderate speed and minimal pressure to engage the teeth, then adjust the rotation speed and pressure of the hole saw as the penetration deepens. The operator may need to also adjust the torque of the drill to overcome the resistance of the hole saw cutting through the fiberglass. Some drills have adjustable speed and torque settings that can be changed to better suit the task at hand. Refer to the drill’s user manual for guidance.
With a new hole saw, the user will not have to apply much pressure when operating. The focus should be on maintaining a moderate drill speed and not allow the drill bit to slip out of the pilot hole. Also, ensure the hole saw is perpendicular to the work surface to prevent the saw from binding. There will be fiberglass dust and it is recommended to wear appropriate eye and respiratory protection. Periodically you may need to temporarily run the hole saw in reverse to clear debris from the teeth.
Figure 4. Aligning the hole saw with the marked center point for the bulkhead fitting (left) and beginning the penetration of the hole saw through the tank wall (right).
The operator should not be pushing the whole saw with great force near the end of the cut. Applying too much pressure will cause “tear out” where the back of the tank wall can be damaged as the metal cylinder of the hole saw pops through the wall. After penetrating through the tank wall, remove the hole saw from the newly created orifice slowly. Generally the hole saw will contain the remnants of the tank body within the metal cylinder (Figure 5). It can be removed by using a flat head screw driver to leverage the piece free. You will end up with a circular piece of the tank in your hand.
Figure 5. Using a flathead screw driver (left) to remove the tank wall remnant (middle) remaining in the hole saw blade (right).
You should remove the remnant piece (Figure 5) from the hole saw before proceeding with additional cuts. A shop vacuum (Figure 6) can be used to clean the fiberglass dust created from the creation of the opening through the tank bottom. The result is a clean hole for the installation of the bulkhead fitting (Figure 6).
Figure 6. Cleaning up the fiberglass dust with a shop vacuum (left) and the resultant orifice created by the hole saw for installation of the bulkhead fitting (right).
Once you cut the appropriate hole size into your tank/vessel, you can then install the bulkhead fitting. The body of the bulkhead fitting will pass through the tank bottom leaving the flange and gasket (Figure 7) seated flush on the inside of the tank. The threaded bulkhead body will be visible on the outside of the tank (Figure 7).
Figure 7. Installing the bulkhead fitting through the tank wall which leaves the flange and gasket contacting the inside of the tank wall (left) and the threaded body passing through the tank wall and visible on the outside of the tank (right).
With the flange and gasket lying flush on the inside of the tank wall and the threaded body exposed on the outside of the tank wall, we can now install the retaining nut (Figure 8). Depending on the bulkhead fitting the nut will need to be threaded in either a clockwise or counterclockwise fashion. You can tighten it by hand until it is secured against the tank wall. But you will need to use a pair of channel locks to give the nut an additional ½-turn to finish the job (Figure 8).
Figure 8. Installing the nut on to the threaded body of the bulkhead fitting (left) and hand tightening the nut snugly against the outside of the tank wall (middle) and then finishing the installation with a pair of channel locks (right).
Things to keep in mind and to avoid during and after installation:
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The gasket and flange of the bulkhead fitting should be installed on the tank’s inside.
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Avoid overtightening the nut. If after tightening the nut, you can visually see the gasket is bulging or squeezing out from under the flange, then you have over-tightened the nut..
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The bulkhead can leak for a number of reasons including not tightening the bulkhead nut sufficiently, the threads of a female PVC fitting joined to the bulkhead are not sealed properly with teflon tape or the PVC pipe and socket end of the bulkhead fitting did not mate well after the application of solvent and cement. In addition, the flange or nut may have cracked from overtightening or shifting/settling of the tank/vessel.
In conclusion, bulkhead fittings provide an excellent means to create an inlet or outlet to a tank and allow a secure, leak free, structure for PVC pipe to connect to. The installation process is often easy to accomplish once the correct sized hole saw is identified.The phrase "measure twice, cut once" means to double-check your measurements for accuracy before making a cut, particularly in plumbing. An unwanted hole in your tank can quickly become a problem!
Feel free to call Aquatic Equipment & Design, Inc. staff at 407-995-6490 or email info@aquaticed.com if you require clarification on the correct choice of bulkhead fitting.