Drain Plugs

Male NPT Threaded Plugs for Closing a Tank Drain or Connection

A threaded plug closes a female NPT connection when it is not being used. On a plastic tank, it is commonly used to close an existing drain or an unused threaded connection.

NPT threads are tapered: the male and female threads tighten progressively as they are screwed together. This type of connection is therefore not intended to be repeatedly screwed and unscrewed like a straight thread, such as the one found on a plastic bottle cap. When a connection needs to be opened frequently, a component designed for that function, such as a valve, should be used instead.

The plugs offered by Agrico Plastics are made of Schedule 80 glass-reinforced polypropylene and have a male NPT thread.

Seven sizes are available: 1/2", 3/4", 1", 1-1/4", 1-1/2", 2" and 3".

The choice may seem simple, but two points must be checked before ordering: the connection diameter and its thread type.

A Male NPT Plug Must Be Installed in a Female NPT Connection

This is the first point to check.

These plugs have a male NPT thread. They are therefore designed to be threaded into a compatible female NPT connection, such as those found on some NPT bulkhead tank fittings.

For example, a 2" male NPT plug must be used with a 2" female NPT connection.

Nominal diameter alone does not confirm compatibility. A 2" connection is not automatically an NPT connection simply because its nominal diameter is 2".

Before ordering, check both the size and the type of connection you need to close.

Closing a Tank Drain

The primary use of these plugs is to close a tank drain when you do not want to install a ball valve or another adapter. The plug can also be used to close another female NPT connection that will remain unused.

The plug threads into an appropriate female NPT connection to close the drain or connection. Even though NPT threads tighten progressively as they are screwed together, an appropriate thread sealant, such as PTFE (Teflon) tape, should still be used when suitable for the application.

A plug does not replace a valve. If the outlet must be opened and closed regularly to drain the tank, a ball valve should be used.

For more information on how drains, fittings and other components relate to tank selection and use, visit our Plastic Tank Applications page, which covers storage, transport, mixing, dosing, chemical, decanting and containment applications.

Plug or Valve: What Is the Difference?

Both may be installed at a tank outlet, but they serve different purposes.

If the drain will be used frequently, repeatedly removing and reinstalling a threaded plug is not appropriate. A valve designed for that function should be installed.

If the liquid will then be transferred to another piece of equipment, a Camlock fitting can also be part of the assembly when quick hose connection and disconnection is required.

The choice therefore depends on what you intend to do with the connection once the tank is in service.

Seven Sizes From 1/2" to 3"

The plugs are available in the following sizes: 1/2", 3/4", 1", 1-1/4", 1-1/2", 2" and 3".

This range covers several common female NPT connection sizes used in tank and piping assemblies.

However, the plug should not be selected according to tank capacity. A larger tank does not necessarily have a larger drain, and two tanks with the same capacity may have different fittings.

Check the actual connection on your tank or installation. Tank selection and the components required for an installation should always be considered together.

Schedule 80 Glass-Reinforced Polypropylene

The plugs in this category are made of Schedule 80 glass-reinforced polypropylene.

The product specifications indicate a pressure rating of 150 PSI at 21 °C.

As with other fittings in an installation, this value does not automatically mean that the complete assembly has the same pressure rating. The bulkhead fitting, valve, other fittings and operating conditions must also be considered.

When a chemical product is used, the compatibility of the liquid with polypropylene and with all other materials in contact with the product must also be verified.

This is particularly important in agricultural applications, where plastic tanks may be used for potable water, fertilizers and other liquids.

Plan for Proper NPT Thread Sealing

An NPT threaded fitting must be assembled correctly to obtain a leak-tight connection.

The plug threads into the female NPT connection with a thread sealant appropriate for the thread and application, such as PTFE (Teflon) tape when compatible with the liquid and operating conditions.

The plug must be properly tightened without unnecessarily forcing the plastic threads. There must also be enough clearance around the plug to use the appropriate tightening tool during installation.

This is particularly important when the drain is located very close to the floor, a support, a wall or another piece of equipment.

Check Why the Connection Was Plugged

When working on an existing tank or installation, a plug may sometimes close a connection that was originally intended for another function.

If you remove the plug, check the diameter and thread type of the new component to be installed, as well as the condition and sealing surface of the existing threaded connection.

Depending on the new assembly, a male x male NPT pipe nipple or another reducing fitting may also be required to connect the components properly.

Frequently Asked Questions

What Size Plug Do I Need for My Tank?

The size depends on the female NPT connection being closed, not on the capacity of the tank. Check the diameter and thread type of the existing connection.

Are the Plugs Male or Female Threaded?

All seven plugs in this category have a male NPT thread. They are designed to thread into a compatible female NPT connection.

Can I Use a Plug Instead of a Valve?

Yes, but only if your objective is to close off the connection. If you need to open and close the drain regularly, a valve is required.

Should I Use PTFE (Teflon) Tape on the Threads?

An NPT connection requires an appropriate thread sealant. PTFE tape can be used when compatible with the liquid and operating temperature.

Can I Select the Plug According to the Size of the Tank?

No. Tank capacity does not determine the drain size. Check the actual connection that needs to be closed.