≤ 200 °C
ESP gas temperature
ESP components · High-voltage isolation
ESP insulators and high-tension conductors
ESP support insulators carry the full weight of the discharge-electrode system while isolating it at negative high voltage from the earthed casing, in flue gas up to 200 °C. Made from high-alumina porcelain, they fail overwhelmingly by surface tracking after condensation — so heaters and purge air, sized for start-up rather than steady state, are part of the insulator system.
150–180 °C
Bagasse-duty design point
≈ 50 %
Bagasse fuel moisture (as fired)
177 kW absorbed
HT system design-basis load
01 — Function
Three insulator duties, one failure mechanism
Everything at high voltage inside the casing hangs, mechanically and electrically, from a few pieces of porcelain.
An electrostatic precipitator holds its entire discharge system — frames, discharge electrodes, bracing — at negative high voltage inside an earthed steel box filled with dusty flue gas at up to 200 °C. Three insulator families make that possible. Support insulators, mounted in roof compartments or a penthouse, carry the full dead weight of the discharge system while providing the electrical isolation. Shaft insulators couple earthed drive machinery to high-voltage internals — typically where rapping systems act on the discharge-frame from outside the casing. Bushings pass the high-tension feed from the transformer-rectifier set through the casing wall or duct.
All three sit within the same electrostatic precipitator design logic: the porcelain is a structural member first and a dielectric second, and its working environment — gas at a 150–180 °C bagasse-duty design point, heavy dust, moisture from fuel fired at roughly 50 % water — is what actually sizes the system. The bulk material is high-alumina porcelain, glazed to a smooth surface: high compressive strength for the hanging loads, high dielectric strength, and tolerance of thermal cycling between ambient and flue-gas temperature at every start and stop.
02 — Failure physics
Tracking, dewpoint and why heaters are sized for start-up
Insulators almost never fail as insulators. They fail as wet, dirty surfaces.
Why do ESP support insulators fail by tracking?
Because a cold insulator below the flue-gas dewpoint collects condensate, the condensate binds dust into a conductive surface film, and leakage current through that film burns carbonised tracks into the glaze. Each track shortens the effective creepage path until the insulator flashes over at working voltage. The porcelain body is rarely the weak point — its surface condition is.
The vulnerable window is start-up. In steady operation the gas itself, at 150–180 °C, keeps every surface in the roof compartment above dewpoint and the heaters barely work. At a cold start the situation inverts: the burner train sends warm, wet gas — bagasse at ~50 % moisture as fired produces a high water dewpoint — into a casing whose porcelain, steel and refractory are at ambient. Everything below dewpoint sweats. This is why insulator heaters, like hopper heaters, are sized for the start-up case and not for steady-state losses: the heater must lift the full cold thermal mass of the compartment above dewpoint, with margin, before the field is energised. A heater sized only to hold temperature in steady running is undersized by a large factor for the duty that actually matters CONFIRM: Arrow standard heater ratings per insulator compartment and required pre-heat time before energisation.
The second defence is purge air: a small continuous flow of filtered, heated air fed into each insulator compartment, holding it at slight positive pressure so dusty flue gas cannot migrate up around the porcelain. Purge keeps the creepage surface clean; heating keeps it dry. Lose either and the leakage current begins its slow work — visible on the field controller as sparking at abnormally low kV long before the flashover.
03 — Hardware
Insulator types, materials and dimensions
Creepage distance is the real specification; everything else follows from it.
Creepage — the distance along the insulator surface from live to earth — is what dust and moisture attack, so insulators are specified by creepage class as much as by height or seat diameter. Classes are selected against gas condition and dust conductivity CONFIRM: creepage distance classes and mm values used in Arrow specifications.
| Type | Duty | Form | Material | Governing dimensions |
|---|---|---|---|---|
| Support insulator | Carries discharge-system weight; main HV isolation | Conical or cylindrical hollow body | High-alumina porcelain, glazed CONFIRM: alumina content grade | Seat diameter, height, wall thickness, creepage class CONFIRM: standard sizes |
| Shaft insulator | Isolates rapper or drive shafts crossing to HV frames | Cylindrical, through-bore | High-alumina porcelain; alloy end fittings | Bore, length, coupling pattern |
| Bushing | HT feed-through at casing or duct wall | Hollow porcelain with HT stud | High-alumina porcelain; silicone gasketing | Flange bolt circle, stud size, creepage class |
| Compartment heater | Holds surfaces above dewpoint at start-up | Rod or finned element | Stainless sheath | Rating per compartment CONFIRM: kW rating |
| Purge-air unit | Positive pressure, clean creepage surface | Fan + filter + heater | Carbon-steel housing | Flow per compartment CONFIRM: purge flow and ΔT |
HT conductors and bus ducts. Between the transformer-rectifier sets and the bushings, the negative HT feed runs either as insulated HT cable or as a rigid bare conductor inside an earthed, weatherproof bus duct. The duct solution dominates on retrofits: it is inspectable, tolerates the 150–180 °C roof environment, and its air clearances are set by geometry rather than by an ageing cable dielectric. Duct sections carry their own small support insulators and drain points, and clearances are dimensioned to the TR set's rated voltage — the design-basis ESP puts 177 kW through this path, so a single tracking joint in the duct can idle an entire bus section.
04 — Failure modes
Failure modes and symptoms
Most insulator faults announce themselves electrically weeks before they fail mechanically.
What are the warning signs of a failing ESP insulator?
Sparking at abnormally low kV — especially in the first hours after a start — rising leakage current, and a field that will not hold its air-load voltage curve. At inspection: grey or black track marks on the glaze, dust bridging across the surface, or condensate staining. Cracked porcelain usually follows earlier electrical symptoms that went unread.
- Surface tracking (condensation + dust film)
- Low-kV sparking after starts, leakage current trending up over weeks. Restore heaters and purge first; replace tracked units — a carbonised track cannot be cleaned back to service.
- Cracked or spalled porcelain
- Sudden flashover or a field that trips instantly. Causes: thermal shock from energising cold, over-torqued mountings, rapping loads through a seized shaft insulator.
- Dust bridging in the compartment
- Gradual kV droop; visible cone of dust over the insulator. Almost always a failed purge fan or filter, not an insulator defect.
- Heater element failure
- No symptom in steady running — then tracking after the next cold or rainy-season start. Test heaters at every outage, not just before monsoon.
- Bus-duct tracking at gaskets and drain points
- Sparking audible at the duct, kV noise on the controller. Inspect joints; renew gaskets and desiccant where fitted.
05 — Replacement
Replacement and interchangeability
Porcelain is a commodity; fit is not.
Arrow Energy Co., Ltd. supplies support, shaft and bushing insulators — with heaters, purge units and HT conductor hardware — as replacements in precipitators built by other OEMs, under its ESP upgrade and spares scope. Interchangeability is decided by a short list of dimensions: seat or flange diameter, bolt circle, overall height, through-bore where applicable, and creepage class. Every third-party supply therefore starts with a dimensional survey of one removed insulator and its mounting, plus the TR set nameplate so electrical duty is matched. Replacements are supplied with new gaskets and, where the survey shows the original heaters were sized for steady state only, with a corrected start-up heater rating and a stated pre-heat procedure.
FAQ
Engineering questions, answered
What are ESP support insulators made of?
High-alumina porcelain, glazed, formed as conical or cylindrical hollow bodies. The material combines compressive strength to carry the discharge-electrode frame, dielectric strength for the negative high voltage, and thermal-shock resistance for flue gas service at up to 200 °C, with bagasse-duty design points typically 150–180 °C.
Why do ESP insulators fail during start-up rather than normal running?
Because start-up is when surfaces sit below the flue-gas dewpoint. Bagasse is fired at about 50 % moisture, so the gas is wet; moisture condenses on a cold insulator, mixes with dust into a conductive film, and leakage current burns carbonised tracks. Heaters must bring surfaces above dewpoint before high voltage is applied.
Why are insulator and hopper heaters sized for start-up, not steady state?
In steady operation gas at 150–180 °C keeps surfaces warm and heaters idle along. At start-up the heaters alone must lift a cold mass of porcelain, steel and refractory above the flue-gas dewpoint before energisation — a far larger duty. A heater sized for steady-state losses will never get a cold ESP safely to voltage.
What does purge air do on an ESP insulator?
Filtered, heated purge air is blown gently through the insulator compartment so flue gas, moisture and dust cannot migrate onto the insulator surface. It maintains the clean creepage path between the high-voltage frame and earth. Loss of purge or heating shows up as rising leakage current and sparking at abnormally low kV.
Can Arrow supply insulators for another OEM's precipitator?
Yes. Support, shaft and bushing insulators are interchangeable if seat diameter, height, bolt pattern and creepage class match, so Arrow takes a dimensional survey of the existing insulator and its mounting before supply. Electrical duty is checked against the TR set rating — 177 kW absorbed power on the design-basis ESP.
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