Why One Size and One Tonnage Are Not Enough to Quote Grate Bars
A typical inquiry reads: grate bars 390 × 55 × 50 mm, for a 15 t/h biomass boiler, please quote. With that much information a supplier can name a rough price for one plausible bar, and nothing more. Whether the price survives contact with the real part depends on five variables that are not in the message.
1. What actually sets the price of a grate bar
Grade. A heat-resisting cast iron such as RQTSi5 and a heat-resisting steel casting such as Cr25Ni20 sit at opposite ends of the cost range, and the alloy content is the main reason. Two bars of identical shape can differ several-fold in price on grade alone.
Weight and section. Price follows poured weight, and section thickness is a design choice, not a given. A bar quoted 10 per cent lighter than another may simply have thinner walls in the same envelope.
Pattern and core. A plain bar needs a simple pattern. A perforated bar needs a core or additional machining; a bar with cooling ribs or an unusual engagement profile needs a more expensive pattern. Pattern cost is amortised over the order, so twenty pieces and five hundred pieces are not the same unit price.
Tolerance and machining. As-cast, machined on the seat faces, or fully machined are three different products with three different prices, and the drawing has to say which.
Set composition. A “set” is rarely one part number: moving bars, fixed bars, side bars, side seals and pins are usually quoted together, and the mix changes the average price per kilogram.
Commercial terms and documents. Packing for sea freight, marking, a chemical analysis report or a spectrum report per heat, and the delivery term all move the number, and they are the first things silently dropped when a quote looks unusually cheap.
2. Same outline, different part
Three bars measuring 390 × 55 × 50 mm can be non-interchangeable. What is missing from a three-number description:
- the engagement profile — the lugs or teeth that carry the load and set the pitch;
- the support face and its angle on the beam or cross beam;
- the pin hole diameter and position;
- wall thickness and the internal cooling ribs, which decide metal temperature more than the grade sometimes does;
- the side faces that have to seal against the neighbouring bar;
- the hole pattern, if the bar is perforated, and its relationship to the wind-box compartments.
Pitch is the one to watch. It fixes how many bars cover a metre of grate width, so a “same size” bar with a different engagement profile changes the piece count, the side seal fit and the price of the whole set. That mechanism is walked through in will holed bars fit an existing grate.
3. The six items that make a quotation possible
| Item | What to send | What it fixes |
|---|---|---|
| 1. Position in the grate | Grate type, and whether the bar is moving, fixed, side, left, middle or right | Which part number and which pattern |
| 2. Geometry | Outline drawing, or length × width × height, pitch, engagement profile, pin holes, seal faces | Whether the part is interchangeable at all, and the pattern cost |
| 3. Quantity | Pieces per set, or effective grate length × width so the count can be computed | Unit price, and whether a core or pattern is justified |
| 4. Material | Grade plus standard and edition, e.g. RQTSi5 to GB/T 9437, ZG40Cr25Ni20 to GB/T 8492 | The alloy content, and therefore most of the price |
| 5. Duty | Fuel type and mix, moisture, particle size, ash and ash fusion, achieved service life, failure photos | Whether the grade is adequate, and whether a cheaper or dearer one is the right answer |
| 6. Commercial | Delivery term, destination, packing, documents required, required date | Freight, packing cost and what arrives with the goods |
Items 1 to 4 are enough for a price. Item 5 is what turns a price into a recommendation. Item 6 is what turns a recommendation into an order that arrives in usable condition.
4. A worked case
In a recent inquiry the buyer had the bar size 390 × 55 × 50 mm at about 5 kg, a set of 540 pieces, and a 15 t/h biomass stepped grate. With those numbers together with a grade, a quotation for a plain bar is straightforward: 540 pieces is about 2.7 tonnes of castings and about 11.6 m² of gross face area, which is a normal figure for that duty.
The same three numbers could not answer the questions that followed — whether holed bars would fit, how many pieces of the new type would be needed, whether efficiency would improve, and what the price difference would be. Every one of those needed item 2 (pitch and engagement profile of the holed type) and item 5 (what the bars were actually failing from), neither of which was in the original message.
5. How to compare three quotations
Suppliers quote different things under the same words. To compare like with like, put five lines into a small table and demand an entry against each:
- grade, plus the standard number and edition it is ordered to;
- weight per piece, weighed rather than nominal;
- tolerance and which faces are machined;
- documents supplied — chemical analysis, spectrum report per heat, dimensional report;
- packing method and what is included in the set.
A quote that cannot fill those five lines is not cheaper; it is unfinished. The most common hidden difference on this product is wall thickness: two bars with the same outline and the same grade can differ by a fifth in mass, and the lighter one will have the shorter life.
6. What we ask for, and why
We supply moving and fixed bars, side seals and matching parts for biomass reciprocating grates, dispatched from a partner factory. Dimensions and material are checked against a drawing or against a measured sample, so the first thing worth sending is a drawing or a measured bar — a photograph is enough to identify the type, not to make a part. With the six items above, a reply can carry grade by position, piece count, unit price and a delivery date in one message.
