When a specification names a particular blower, a supplier can quote exactly that and also point out a better alternative. In an elevated flare tender in North America, the supplier offered a compliant blower that surges at 25 to 30% of maximum flow, and its own standard backswept blower that does not surge from full flow to zero. This post explains why that matters.

The challenge: a specification that locks in the blower

The specification called for an open-flame elevated flare rated at 90 million Btu per hour, about 2,750 scfm of 50% methane gas, with a 6:1 turndown. The flare stack, 20 ft to the tip in 310 stainless steel, was suitable for continuous use above 850°C and a design life of at least 15 years. It also called for a particular make of blower rated at 2,750 scfm and 40 inches water column, and a propane pilot.

The approach: a compliant bid and a flagged alternative

Compliant bid

The supplier quoted what was asked. That meant the named blower and a propane pilot, with a note that these were for the compliant bid. It added that a propane pilot is not required, because landfill gas methane can serve as the pilot, and offered a discount if the requirement were reversed.

The alternative

The standard blower has a fully stainless steel casing and impeller and backswept blades. It does not surge at any point from 2,750 scfm to zero flow. A blower built to the specification as written surges at about 25 to 30% of its maximum flow. It also uses direct drive, so no belt maintenance. A discount was offered if the supplier’s blower were adopted.

Why surge matters

Surge is unstable operation at low flow, with pulsing and vibration. A landfill gas system often runs at part load, so a blower that surges forces the operator to avoid part of its range, or to recirculate gas. A non-surging blower can follow the gas field down, including at pilot gas flow.

The rest of the package

• Manual flame temperature control to 850°C minimum, with automatic as an option.
• Pressure head: the supplier’s experience is that 100 to 150 mbar suits most collection networks.
• A PLC with an auto-dialer, industrial computer interface, paperless chart recorder and modem.
• A condensate injection system that feeds knockout pot condensate into the flare shroud while a flame is present.
• Structural calculations signed by a registered engineer, and a stack located with outriggers, not guy ropes.

What to consider for your own project

1. Ask for the operating range of the blower, not just the duty point.
2. Keep alternatives visible. A compliant bid plus a better option lets the client choose.
3. Question bottled pilot fuel if methane can do the job.
4. Request the structural calculations for tall stacks.

FAQ

What is blower surge?

Unstable pulsing at low flow when a blower operates to the left of its stable curve.

Can a flare be lit without propane?

Yes, where the landfill gas methane is strong enough.

Reviewing a specification?

Send it over. See also sizing the gas booster.