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OSV & check valve location on oil piping - Webster Technician's Manual p. 122Oil Piping Check Valves - Non Return Valves
Uses, locations, prohibitions & codes for the use of check valves in oil piping systems

Oil piping check valve / non-return-valve use, installation & prohibitions: this article describes the use of check valves in heating oil piping systems. We explain where some manufacturers recommend that check valves should be installed, what the check valve is accomplishing.

We point out that other heating equipment and oil burner or fuel unit manufacturers prohibit using a check valve at all. We include citations from key industry sources & building codes on the use of oil piping check valves including Suntec & Webster.

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Oil Piping Check Valves - Non Return Valves

Oil tank foot valve - Envirosafe Where & When do we Put an In Line Check ValveOn a Heating Oil Line? Ever?

Check valves are recommended for use with home heating oil (No 2 fuel oil) on the oil supply line (the suction line) just above the oil tank in some heating oil pumping & piping installations in order to keep oil in the pipeline and to make service of a foot valve in the oil tank a bit easier.

Check valves are also used on No. 6 fuel oil systems, but at the oil pump as drain-back problems are less troublesome with heavier grade fuel. - Pump School (2012)

Shown at left is an oil tank foot valve, a check valve used to help prevent the loss of prime in the oil suction line. This valve is installed at the end of the suction line in the heating oil tank.

[Click to enlarge any image]

The use of check valves on heating oil piping return lines has been found on some heating systems to prevent a messy oil spill from the return line and to make service of the fuel unit easier (Webster 2014). Check valves are described for this use to steer oil piping installers away from installing a fusible link type OSV on the return oil line - a possible cause of a fire disaster discussed separately

at OIL SUPPLY LINE SAFETY VALVES, OSVs.

Watch out: while some manufacturers' literature (Webster) describes where and why a check valve may be installed on a heating oil system pipeline, other equipment manufacturers (Suntec) and sources prohibit their use.

[Shown below is Beckett Corporation's oil line check valve. Click to enlarge any image.]

Beckett oil line check valve - Beckett Corporation

The worry is an oil-entrapment problem between the fuel unit and the return pipe that, combined with thermal expansion, is reported to cause return oil line leaks or leaks at the fuel unit itself. We have also read a claim that at fuel unit shut down the check valve could "blow the pump seal" - an explanation by Suntec.

If two oil lines are used to supply an oil burner, (a supply and a return) install an oil safety valve or OSV or fusible link oil line shutoff valve only on the oil supply line at the oil pump on the oil burner. Do NOT install an automatic oil line shutoff on the return oil line between the oil burner and the oil tank.

If a protection against oil back-flow at the return line is a concern, and if the manufacturer permits, use a check valve instead. Check valves like this one permit oil to flow just in one direction. They do not close down in event of a fire. Installed on the oil return line a check valve permits oil to flow from the oil pump in one direction only: back to the oil tank.

The check valve on the oil return line, like the OSV on the oil supply line, prevents oil from spilling back out of the oil piping should the service technician need to disconnect the oil piping from the fuel unit - for example during fuel unit servicing or repairs, or during service of a backup oil pump on larger and commercial systems.

OSV & check valve location on oil piping - Webster Technician's Manual p. 122

Check valves suitable for placement on the oil return line are available from various oil heat equipment producers including Webster.

As Webster points out, check valves in the suction line between the oil tank and their SPM units assures that the oil line and pump are always full of oil, supporting fast oil burner starts.

[Click to enlarge any image]

You will find this arrangement more likely on commercial and multiple-burner installations such as we illustrate in Webster's sketch, adapted and annotated at left. [7a] There the oil supply coming from the oil tank is marked in green, passing through an oil pump, a check valve (red) and on to three oil burners through three OSVs (blue).

The oil return line is marked in light brown; you will see a second check valve (red) on the return line.

Oil Line Check Valve Mistakes & Warnings

Watch out: do not install a check valve on the inlet of a 1-pipe oil system and do not install a check valve on the return line of a two pipe oil system (as we just described above) IF the oil burner or fuel unit manufacturer recommends against it. Example: R.W. Beckett, in describing the installation of their Cleancut Single Stage or 2-Stage Fuel Units (Part No. 21844 and 21941 respectively) warns:

Do not use a check valve in the inlet line of a 1-pipe system (with or w/o a boost pump), or in the return line of a 2-pipe system.

NFPA 31-25, 8.5 states that ‘A return line from a burner or pump to a supply line shall have no valves or obstructions and shall enter the top of the same tank.’ Check valve flow restriction in a return line

Check valve flow restriction in a return line can elevate pressures and damage the fuel unit seals.

Dangerous thermal expansion of oil trapped by an inlet line check valve can create extreme pressures that damage fuel unit seals, line fittings, inlet filters, gauges and other components.

A properly installed vacuum safety valve, such as Suntec PRV-38, having having accumulator effect and pressure relief to tank is acceptable in the inlet line. [See Oil Entrapment Safety Warning below - Ed.] - R.W. Beckett [2a]

When Do We Use or Not Use a Check Valve on Oil Line Piping?

One manufacturer, Webster, explains where & why to use check valves on an oil line, and oil piping texts (NYFD 2012) also refer to their use to keep an oil piping header full or to prevent siphonage, while other manufacturers, Beckett and Suntec, indicate that we don't use them (at least with a particular oil burner model, or according to citation of a poorly-worded NFPA 31-25 8.5 that is confusing itself). So do we install a check valve or not?

And weighing in, some heating equipment manufacturers say check valves are not necessary.

Each oil burner should have its own suction line. A common return line can be used as long as the diameter is large enough. Check valves are not required on properly installed systems. Service on fuel units should not be attempted without a suitable vacuum and pressure gage.

The information presented here is intended as a guide only. For piping system design data, consult the installation instructions from the pump manufacturer. - Newmac Boiler Installation Instructions [17]

Suntec Advice on Avoiding Use of Check Valves on Oil Piping

[Uses of Oil Piping] Check Valves, Other Valves & Thermal Expansion Check Valves

A properly-installed fuel oil heating system does not require check valves for proper operation. However, check valves are often used to compensate for deficiencies in oil line piping. For example:

a. Not having the return line submerged in the oil in the tank

b. Having vacuum leaks in the line due to using compression fittings, bad flare fittings, porous fittings, loose fittings or leaky lines.

See OIL TANK PIPING & PIPING DEFECTS for a table of normal running vacuum levels in heating oil piping installations.

Disadvantages [of installing check valves on oil piping]

Check valves increase the amount of vacuum the fuel unit must overcome to supply oil to the system and reduce the distance supply lines can be run.

When installed near the fuel unit, check valves cause turbulence and stripping of air from the oil, resulting in dirty and/or noisy combustion.

Do not use check valves in gravity feed (tank above pump) single-pipe systems, or in a system supplied by a boost or transfer pump. Thermal expansion can cause serious problems.

Thermal Expansion [Problems & Check Valves - aka "Oil Entrapment" Problem]

Thermal expansion is a phenomenon in which a fluid increases in volume when heated.

When fuel oil is heated from 40°F to 70°F, it will increase in volume by 1 - 1/2%. Since oil will only compress at a rate of 1/10%, thermal expansion will greatly increase the oil pressure in a closed system.

In a typical single-pipe system, an air pocket within the pump serves as a cushion against changes in pressure. In a gravity feed- or boost pump-supplied system, the air cushion is eventually absorbed, creating a hydraulically "hard" or closed system which is unable to absorb pressure increases. - "Check Valves, Other Valves, and Thermal Expansion", Suntec, Fuel Oil Pump Service Manual [8][8a][9]

Suntec continues to point out that the effects of thermal expansion are leaks at joints, fittings, pressure gauges, filters, and fuel unit seals. The thermal expansion problem is most-likely in single pipe oil systems, in dual-fuel heating systems during alternate fuel use, and where the ambient temperatures are higher than the fuel temperatures - something particularly likely when pumping heating oil from an underground oil storage tank or after a fresh fuel delivery in winter.

Really? OK so this is confusing, right? Field technicians are obligated to respect the advice of the experts at the equipment manufacturer - in this case Suntec. But the applicability and use of check valves on oil piping installations may also depend on the installation particulars; some of the literature appears to contradict itself most likely because the manufacturers are describing oil piping on different types of systems: residential, commercial, and multiple heater applications.

Really? Suntec recommends use of an oil line pressure relief valve rather than a check valve if there is a problem on a particular oil piping installation - advice that does not address service convenience when working on a fuel unit connected to lengthy oil return piping that may perhaps even run overhead in some buildings.

Here's the bottom line:

Of critical importance is the avoidance of any potential blockage on the oil return piping (such as by installing an OSV on the return pipe or by installing a check valve where one is not recommended by the heater or equipment manufacturer) - because we don't want to blow an oil line, creating a leaky mess (due to oil entrapment), blow the seals in a fuel unit, nor feed a fire by an oil line blow-out during a fire.

  • Thanks to reader Anonymous by request 2/23/2014, for requesting clarification of the safety hazards involved in placing an OSV on the return line of a two-pipe oil system.

    • More Check Valve Oil Piping Warnings

    Watch out: where multiple heating appliances are fed from a common supply and return oil piping loop, a check valve is installed on the return line at each oil burner. [7a]

    Watch out: some fuel unit manufacturers whose oil pumps include an internal check valve (Webster SPM series for example) may advise against installing extra external check valves on the systems where their equipment is used.

    Watch out: Oil Entrapment Safety Warning:

    Webster's Service Technicians Manual describes a potentially dangerous oil entrapment problem as follows:

    In [ heating oil ] supply systems having a check valve on the supply pump discharge, an entrapment condition may exist between the supply pump and OSV®. In the burner OFF condition, cold oil in the line expands when heating to room temperature, causing a line pressure increase that may prevent the OSV® from operating, or may cause leakage at the threaded joints.

    The condition may be observed by use of a pressure gauge at the inlet side of the OSV®, or on the discharge side of the supply pump check valve. Correction requires use of a pressure relief valve as shown in Figure 3 [Illustrated and annotated at above left] , set to insure a maximum 60 PSI at any OSV® inlet under all conditions. - Webster [7a]

    Oil line check valve (C) Daniel Friedman

    Watch out: don't confuse an OSV with a check valve: the valve shown at left should not be used as a check valve in the application we just discussed. This is a Webster OSV & requires a vacuum to open.

    If installed in the return line the valve would be pressurized, not open, cause the shaft seal to rupture or blow out & result in a major oil leak.

    Thanks to reader Rick Johnston for adding clarification.

    In contrast, Webster's Vacuum Breaker (P/N 48598) can be used in the oil suction line as a check valve. - Webster [7a p. 129]

    Oil Line Check Valve Brands, Sources, Manufacturers

    Oil Line Check Valve Troubleshooting & Diagnosis

    When diagnosing high or abnormal vacuums (over 15 inches) in the oil supply line ahead of the fuel unit, don't forget to consider a debris-clogged or stuck check valve. Conversely, abnormally low vacuum may indicate an air leak in the oil piping system.

    If the oil supply appears blocked and there is oil in the tank, in addition to checking for a blocked oil line or a debris-clogged valve or a sticking check valve, double check each check valve to be sure that it has been installed in the right direction. OSVs and check valves should have an arrow embossed on the valve body indicating the intended direction of oil flow through the valve, or some products may be embossed with an "IN" and "OUT" indication.


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