Weber control valve and burner metering orifice theory and data

[SteveRichards]

I am in the process of converting a Summit 650 from propane (LP) to natural gas (NG).

In order to do this I had to reverse engineer the control valve and burner metering orifices so I thought I would share what I learned. The valve in this post is one of the 6 main burner control valves on a LP Summit 650. All the Weber burner control valves operate on the same principle.

Weber BBQs (Genesis and Summit) have 2 components that control the flow of gas (LP or NG) downstream from the regulator - the burner control valve and the burner metering orifice.

Note: some NG Weber BBQs, such as certain Genesis 300 models do not have a regulator. The burners in these models run on the house natural gas pressure - 7" WC. Some Weber NG BBQs do have a regulator and all Weber LP BBQs do.

The burner control valve consists of the body and a valve spool that get rotated when the burner knob is turned. There is also the knob assembly and ignition spark generator, but I am ignoring them in this document because they don't have anything to do with controlling the gas flow for the burner.

The purpose of the control valve is to allow the user to modulate the gas flow to the burner, from low heat to high heat.

This is a burner valve body.



The valve body has one hole underneath connected to the manifold from which the gas comes into the valve. It has one hole going out the back that the burner metering orifice screws into that goes inside the burner itself.

The valve body has a taper that the valve spool sits in. This taper is very precise and the valve spools are probably lapped to each of the valve bodies, so don't interchange valve spools and valve bodies. Keep them together as a pair.

 

[SteveRichards]

This is the valve spool.






The purpose of the valve spool is to feed the correct flow of gas into the burner orifice depending on the position of the control knob.

The valve spool has 3 modes: choked flow, full flow and ignition flow.

The valve spool has 3 different metering features: the slot and poppet valve, the hole and the orifice.

1) Choked flow

When the control valve is in the non lighting, low heat position the orifice hole will be lined up with the gas inlet in the valve body. The orifice will reduce the gas flow through the control valve, thus lowering the heat produced by the burner.

2) Full flow

When the control valve is in the non lighting, high heat position, the open hole will be lined up with the gas inlet in the valve body. The open hole will allow the full gas flow to reach the burner metering orifice, thus allowing the burner to produce full heat.

2b) Medium flow

When the control valve is in the non lighting, medium heat position, the open hole will be partially overlapping the gas inlet in the valve body. This will constrain the gas flow somewhat compared to the full flow setting but not as much as the low heat setting where all the gas flows through the orifice hole. This will produce medium heat.

 

[SteveRichards]

3) Ignition flow

When the burner knob is pushed in to light the burner via an integral spark generator, a poppet valve in the middle of the valve spool opens up allowing gas to flow from the slot in the valve spool to the burner metering orifice, unrestricted. As soon as the control knob completes the striking of the spark generator, the knob pops out, closing the poppet valve and forcing the gas to flow through the metering holes as described above.

The valve spools on BBQs that do not have integral spark generators on the knob will not have the slot nor the poppet valve. They will only have the 2 metering holes.

This is what the end of the valve spool poppet valve looks like:




The Burner Metering Orifice

As we saw above, when the control valve is on high or during the ignition process, the gas flows freely through the valve with little restriction. In order to deliver the proper amount of gas to the burner, the burner metering orifice is installed in the end of the control valve. The size of the hole in the metering orifice determines how much gas gets delivered to the burner.

This is the burner metering orifice:



The 0.94 is for 0.94mm, which is 37 thou, the diameter of the orifice hole. This is also known as a #63 orifice, named for the number 63 drill bit that is the same diameter, 0.037".

Burner metering orifice installed in the end of the control valve body. It goes right inside the end of the burner.

 

[SteveRichards]

The main burners on a Weber Summit 650 are rated at 10,000 BTU/hr each. The propane regulator sets the pressure of the gas to 11" WC to the control valve manifold. The control valve provides almost no resistance to the gas when it is in the high position.

Propane has an energy density of 2516 BTU/ft^3 (CF). 10,000 BTU/hr/2516 BTU/ft^3 = 3.990 CFH.

If we look up the flow of propane at 11WC for a #63 orifice, we get a flow of 4.346 CFH.

Appendix 2: Imperial Orifice Capacity Table (flow values in CFH) – Block E: Fuel Gas Systems

opentextbc.ca

Why is the value we looked up in the table (4.346 CFH) larger than the rated flow rate (3.990 CFH) ? 2 reasons.

1) Orifices have what is called a K value that correlates how much they flow versus how much they should flow. The K value is determined by things like the smoothness of the orifice bore, the length of the bore, the roundness of the edges of the bore, etc. The table we used assumed a K value (orifice factor) of 0.90. It is possible that the orifices that Weber uses have a lower K value.

2) The table assumes that the gas (propane) pressure is 11" WC at the orifice. However, there will be some pressure loss from the regulator to the orifice as the gas travels through the hose, control manifold, control valve, etc. In actuality, the pressure at the burner metering orifice will be less than 11" WC, so maybe Weber accounted for that by installing a larger orifice than what the table indicates what might be needed.

The difference between the theoretical flow rate and the flow rate of the orifice actually installed is about 9%.

 

[SteveRichards]

Natural gas has an energy density of 1030 BTU/ft^3, compared to 2516 BTU/ft^3 for Propane. Thus a burner will need to flow roughly 2.5x more gas to make the same heat when running on natural gas compared to propane. This means that the NG orifices need to be much larger than the propane orifices.

To convert a BBQ from propane to natural gas, one must do 2 things:

1) Enlarge the burner metering orifice to flow the correct amount of natural gas
2) Enlarge the valve spool orifice so that the flame can be modulated to low heat when necessary.

The Summit 650 LP valve spool orifice is about the same size as the burner metering valve orifice, or slightly smaller. It will be desirable to keep the same size ratio when the valve spool is used for NG. In theory one could use the same spool valve orifice size for NG as LP but the burner flow would be very low when on low. Possibly too low to keep the flame going properly.

10,000 BTU/hr / 1030 BTU/ft^3 = 9.7 CFH. 9.7 CFH + 9% = 10.582 CFH.

If the NG supply pressure to the BBQ is 7" WC, the burner metering orifice will need to be #55 or 0.052" to flow 10.828 CFH. The spool valve orifice will need to be a similar size in order to properly meter the NG on low setting.

Appendix 2: Imperial Orifice Capacity Table (flow values in CFH) – Block E: Fuel Gas Systems

opentextbc.ca

I happen to have a NG Genesis 300 BBQ that doesn't have a regulator. I will disassemble a control valve on it and share pictures and the measurements on it.

Stay tuned.

 

[SteveRichards]

I disassembled my NG Genesis 300 main burner control valve.

Here are the NG Genesis (top) and LP Summit (bottom) main burner metering orifices for comparison. They both deliver 10K BTU/hr to the burner.



The first thing to note is that the Genesis metering orifice is much larger overall. The NG Genesis also has a much larger orifice hole than does the LP Summit. This is because NG needs to flow ~2.5x more volume than LP to deliver the same BTUs.

The Genesis NG metering orifice size is a loose #54. #53 won't go in. In the previous post I calculated it would need to be a #55 (smaller than #54).

#55 is 0.0520" = 1.320mm = 10.828 CFH <- by my calculation.
#54 is 0.0550" = 1.397mm = 12.113 CFH <- slightly loose.
#53 is 0.0595" = 1.511mm = 14.176 CHH <- too big.

All flow rates at 7" WC using https://opentextbc.ca/plumbing3e/back-matter/imperial-orifice-capacity-table-flow-values-in-cfh/

The required gas flow for NG is 2.5x greater than for LP for the same heat output. I'm guessing that the NG orifice needs to be slightly bigger than what I calculated in my previous post because of the greater pressure loss in the hose, manifold and valve spool due to the higher flow rate required for NG compared to LP.


Here are the LP Summit(left) and NG Genesis (right) metering valves for comparison.



As expected, the NG Genesis valve spool has a much larger orifice hole than does the LP Summit. This is because NG needs to flow ~2.5x more volume than LP to deliver the same BTUs.

The orifice hole in the NG Genesis spool is a perfect #56 fit. This is just a little bit smaller than the burner metering orifice (#54)

Note that the Genesis valve spool does not have the slot at the bottom of the spool like the Summit does nor the poppet valve. Those 2 features are used for the ignition sequence because the Summit knobs have the spark generators built into them.

The NG Genesis valve does have a metering groove between the orifice and the full flow hole that the Summit doesn't have. This is probably because the rotation between low and high is much longer on the Genesis. The rotation between low and high is much shorter on the Summit because part of the knob rotation is used to actuate the spark generator to light the burner. The gas metering will be less precise with the Summit spool compared to the Genesis spool because of the lack of a groove and less rotation. But that will be the case whether the spool is for LP or NG.

The angle between the orifice hole and the full flow hole on the Genesis is 180 degrees. On the Summit it is a bit less than 90 degrees. The size of the full flow holes is similar on both spools.

In order for the Genesis spool to allow gas to pass through such a big angle, it needs to groove between the full flow hole and the orifice hole.

The holes in the Summit spool, on the other hand, are bridged by the inlet port in the valve body such that at least one of the holes in the spool are in contact with the inlet port at all times. Partial flow metering is handled by the full flow hole overlapping partially with the inlet port in the body. This probably makes the partial flow metering of the Summit valve less precise than the Genesis spool.

The orifice sizes mentioned in this post are for use without a NG regulator, with house gas running at 7" WC. These orifice sizes will not work for a NG BBQ that uses a regulator that runs at a lower pressure.