Corrosive service is the single most common reason braided gland packing fails prematurely: a packing qualified for steam at 250 °C will dissolve in a 10 % hydrochloric acid line within a week if the buyer only asks for "PTFE packing" without disclosing the fluid [S2]. The fix is a six-field spec sheet that pins fluid chemistry, temperature, pressure, shaft speed, stuffing-box geometry, and material construction before the RFQ goes out.
On 2026-05-24 wholesale listings on Made-in-China.com show standard gland packing priced at US$ 1.00–15.00 per kg (50 kg MOQ) and Kevlar-aramid packing at US$ 10.00–29.40 per kg (2–100 kg MOQ), with lead-time and certification documents driving the spread rather than the raw fibre [S4][S6]. That pricing spread is the commercial evidence that material construction — not just "PTFE" or "graphite" — is what the supplier is being paid to engineer.
Fluid Chemistry: The Field That Decides Everything Else
Compatible pH range is the first decision filter, because pH 0–14 coverage does not exist in a single yarn. PTFE (expanded or braided) is the default for strong oxidising acids and caustics because PTFE carries the broadest pH 0–14 chemical resistance among common packing fibres [S2]. Graphite braided packing, including expanded graphite reinforced with wire mesh or Inconel wire, handles hot concentrated acids and hydrocarbons where PTFE's 260 °C ceiling is approached [S2].
For mixed-acid or solvent service where neither pure PTFE nor pure graphite lasts, the spec writer should call out "PTFE-impregnated aramid corner, PTFE body" or "graphite-impregnated PTFE yarn" — the construction, not the brand. Aramid-only packing (Kevlar) is reserved for mildly corrosive slurries and abrasive services where wear resistance dominates over chemical attack, and is typically limited to rotating speeds around 20 m/s and pressures of 25 bar on pumps and reciprocating equipment [S3].
Temperature, Pressure, and Shaft Speed: The Three Mechanical Locks
Each yarn family has a published operating envelope that the RFQ must respect. PTFE braided packing is generally rated to roughly 260 °C continuous service, with a pressure ceiling that drops sharply as shaft speed rises [S2]. Aramid gland packing published on Made-in-China.com lists rotating and reciprocating pressure ratings of 25 bar, a valve pressure rating of 250 bar, and a linear surface speed of 20 m/s [S3]. Graphite packing, by contrast, handles higher temperatures but suffers galvanic attack if the gland is flooded with certain oxidisers, which is why reinforced graphite with metal-wire core is sold for steam and high-temperature hydrocarbon service rather than free-flowing acid.
Buyers who omit shaft speed almost always get a packing that runs hot and glazes within days on a centrifugal pump, because frictional heat at the sleeve scales linearly with surface speed. The RFQ line should therefore state: shaft diameter (mm), RPM or linear surface speed (m/s), and whether the equipment is a pump, agitator, or valve — pump sleeves and valve stems have different packings because valve service is static, not dynamic.
Stuffing-Box Geometry: The Field That Forces 70 % of Requotes
Most gland-packing requote cycles are triggered by missing cross-section and depth dimensions. A complete RFQ line carries: stuffing-box bore (mm), packing ring cross-section (mm), number of rings, gland follow-up travel (mm), and whether the gland is packed from the shaft side (standard) or atmospheric side (for toxic fluids, to keep the lantern ring accessible for flush water) [S2].
The cross-section must be chosen first, because the rings are sold in standard square sizes (typically 3, 4.5, 5, 6, 8, 9.5, 10, 12, 14, 16, 19, 22, 25 mm) and the bore is fixed by the housing. Common engineering practice is to size cross-section to roughly 1.5× the shaft diameter, but the gland is built to a specific dimension, so the buyer must read the drawing — not estimate.
Material Construction: Yarn, Braid, Lubricant, and Corner Treatment
A braided gland packing spec line should name four things: corner yarn, body yarn, braid geometry (interlock vs. braid-over-braid), and impregnation. PTFE corner with aramid or cotton body, PTFE-impregnated throughout, is the most common chemical-service build [S3]. For severe acid at high temperature, expanded graphite tape wrapped over a PTFE core is a separate construction, not a grade of PTFE packing, and it must be specced as such [S2].
Kevlar (aramid) packing shows a clear price spread of US$ 10.00–29.40 per kg depending on braid and impregnation [S6], and this is the cheapest construction for mildly corrosive, high-wear service. For a fully specified RFQ the buyer should also call out: halogen content (low-halogen or halogen-free for oxygen service), NACE MR0175 compliance if the line is sour-service hydrocarbon, and any FDA or food-grade requirement if the line is a chemical-intermediate feed to a pharmaceutical plant [S2].
RFQ Comparison: Three Builds Against Four Decision Criteria
Three packing constructions dominate corrosive-line RFQs, and the buyer's choice is driven by four criteria: chemical compatibility, temperature ceiling, shaft speed, and unit cost. Pure PTFE braided packing scores high on chemical compatibility, sits at the 260 °C ceiling, holds to roughly 12 m/s on pumps, and lands at the low end of the US$ 1.00–15.00/kg band [S2][S4]. Aramid-Kevlar with PTFE impregnation scores lower on chemical compatibility (degraded by hot concentrated acid), pushes higher on wear life, reaches the 20 m/s benchmark, and sits in the US$ 10.00–29.40/kg band [S3][S6]. Reinforced graphite with Inconel wire scores high on temperature, neutral on most acids, but fails under strong oxidisers and costs more than both of the above [S2].
For an HCl or H2SO4 line below 200 °C, PTFE braided is the default. For an abrasive slurry with mild chemistry, aramid-Kevlar dominates. For a high-temperature hydrocarbon line above the PTFE ceiling, reinforced graphite is the only credible option. The mistake to avoid is the hybrid spec "PTFE/graphite" written without naming which yarn is the corner and which is the body — every supplier will interpret that differently.
What the RFQ Must NOT Omit: Flush, Cooling, and Emissions Fields
A complete RFQ for a corrosive line includes the flush plan, because most gland-packing failures in chemical service are thermal, not chemical. The buyer should specify whether the gland is fed with a flush liquid (water, solvent, or barrier fluid), whether a lantern ring is required to allow flush injection between rings, and whether a cable gland-style segmented seal is being considered as a packing alternative for fugitive-emissions-tight service [S2].
Buyers writing valve RFQs alongside the pump RFQ should read the gate-valve spec map for sanitary process lines and the needle-valve spec for steam condensate lines, because the packing envelope on a valve stem is different from a pump shaft and the same fluid-service rules apply. Two trackable signals for the next RFQ cycle: suppliers who answer with a single line price instead of a construction breakdown should be eliminated, and any quote that does not list corner yarn, body yarn, impregnation, and ring count is incomplete by definition.
For the relevant spec sheets and selection criteria, see fluid coupling.