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
- Ask for the operating range of the blower, not just the duty point.
- Keep alternatives visible. A compliant bid plus a better option lets the client choose.
- Question bottled pilot fuel if methane can do the job.
- 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.