  Sulphur is an important, volatile alpha element but its role in the Galactic
chemical evolution is still uncertain. We derive the S abundances in RGB stars
in three Galactic globular clusters (GC) that cover a wide metallicity range
(-2.3<[Fe/H]<-1.2), namely M4, M22, and M30. The halo field stars show a large
scatter in the [S/Fe] ratio in this metallicity span, which is inconsistent
with canonical chemical evolution models. To date, very few measurements of
[S/Fe] exist for stars in GCs, which are good tracers of the chemical
enrichment of their environment. However, some light and alpha elements show
star-to-star variations within individual GCs and it is yet unclear whether
sulphur also varies between GC stars. We used the the infrared spectrograph
CRIRES to obtain high-resolution (R~50000), high signal-to-noise (SNR~200 per
px) spectra in the region of the S I multiplet 3 at 1045 nm for 15 GC stars
selected from the literature (6 stars in M4, 6 stars in M22 and 3 stars in
M30). Multiplet 3 is better suited for S abundance derivation than the more
commonly used lines of multiplet 1 at 920 nm, since its lines are not blended
by telluric absorption or other stellar features at low metallicity. We used
spectral synthesis to derive the [S/Fe] ratio of the stars assuming local
thermodynamic equilibrium (LTE). We find mean [S/Fe] = 0.58 +/- 0.01 +/- 0.20
dex (statistical and systematic error) for M4, [S/Fe] = 0.57+/-0.01+/-0.19 dex
for M22, and [S/Fe] = 0.55+/-0.02+/-0.16 dex for M30. The negative NLTE
corrections are estimated to be in the order of the systematic uncertainties.
With the tentative exception of two stars with measured high S abundances, we
conclude that sulphur behaves like a typical alpha element in the studied
Galactic GCs, showing enhanced abundances with respect to the solar value at
metallicities below [Fe/H] = -1.0 dex without a considerable spread.
